Provided by: samtools_1.16.1-1_amd64 
NAME
samtools-mpileup - produces "pileup" textual format from an alignment
SYNOPSIS
samtools mpileup [-EB] [-C capQcoef] [-r reg] [-f in.fa] [-l list] [-Q minBaseQ] [-q
minMapQ] in.bam [in2.bam [...]]
DESCRIPTION
Generate text pileup output for one or multiple BAM files. Each input file produces a
separate group of pileup columns in the output.
Note that there are two orthogonal ways to specify locations in the input file; via -r
region and -l file. The former uses (and requires) an index to do random access while the
latter streams through the file contents filtering out the specified regions, requiring no
index. The two may be used in conjunction. For example a BED file containing locations
of genes in chromosome 20 could be specified using -r 20 -l chr20.bed, meaning that the
index is used to find chromosome 20 and then it is filtered for the regions listed in the
bed file.
Pileup Format
Pileup format consists of TAB-separated lines, with each line representing the pileup of
reads at a single genomic position.
Several columns contain numeric quality values encoded as individual ASCII characters.
Each character can range from “!” to “~” and is decoded by taking its ASCII value and
subtracting 33; e.g., “A” encodes the numeric value 32.
The first three columns give the position and reference:
○ Chromosome name.
○ 1-based position on the chromosome.
○ Reference base at this position (this will be “N” on all lines if -f/--fasta-ref has not
been used).
The remaining columns show the pileup data, and are repeated for each input BAM file
specified:
○ Number of reads covering this position.
○ Read bases. This encodes information on matches, mismatches, indels, strand, mapping
quality, and starts and ends of reads.
For each read covering the position, this column contains:
• If this is the first position covered by the read, a “^” character followed by the
alignment's mapping quality encoded as an ASCII character.
• A single character indicating the read base and the strand to which the read has been
mapped:
Forward Reverse Meaning
───────────────────────────────────────────────────────────────
. dot , comma Base matches the reference base
ACGTN acgtn Base is a mismatch to the reference base
> < Reference skip (due to CIGAR “N”)
* */# Deletion of the reference base (CIGAR “D”)
Deleted bases are shown as “*” on both strands unless --reverse-del is used, in which
case they are shown as “#” on the reverse strand.
• If there is an insertion after this read base, text matching
“\+[0-9]+[ACGTNacgtn*#]+”: a “+” character followed by an integer giving the length of
the insertion and then the inserted sequence. Pads are shown as “*” unless --reverse-
del is used, in which case pads on the reverse strand will be shown as “#”.
• If there is a deletion after this read base, text matching “-[0-9]+[ACGTNacgtn]+”: a
“-” character followed by the deleted reference bases represented similarly.
(Subsequent pileup lines will contain “*” for this read indicating the deleted bases.)
• If this is the last position covered by the read, a “$” character.
○ Base qualities, encoded as ASCII characters.
○ Alignment mapping qualities, encoded as ASCII characters. (Column only present when
-s/--output-MQ is used.)
○ Comma-separated 1-based positions within the alignments, in the orientation shown in the
input file. E.g., 5 indicates that it is the fifth base of the corresponding read that
is mapped to this genomic position. (Column only present when -O/--output-BP is used.)
○ Additional comma-separated read field columns, as selected via --output-extra. The
fields selected appear in the same order as in SAM: QNAME, FLAG, RNAME, POS, MAPQ
(displayed numerically), RNEXT, PNEXT.
○ Comma-separated 1-based positions within the alignments, in 5' to 3' orientation. E.g.,
5 indicates that it is the fifth base of the corresponding read as produced by the
sequencing instrument, that is mapped to this genomic position. (Column only present
when --output-BP-5 is used.)
○ Additional read tag field columns, as selected via --output-extra. These columns are
formatted as determined by --output-sep and --output-empty (comma-separated by default),
and appear in the same order as the tags are given in --output-extra.
Any output column that would be empty, such as a tag which is not present or the
filtered sequence depth is zero, is reported as "*". This ensures a consistent number
of columns across all reported positions.
OPTIONS
-6, --illumina1.3+
Assume the quality is in the Illumina 1.3+ encoding.
-A, --count-orphans
Do not skip anomalous read pairs in variant calling. Anomalous read pairs are
those marked in the FLAG field as paired in sequencing but without the properly-
paired flag set.
-b, --bam-list FILE
List of input BAM files, one file per line [null]
-B, --no-BAQ
Disable base alignment quality (BAQ) computation. See BAQ below.
-C, --adjust-MQ INT
Coefficient for downgrading mapping quality for reads containing excessive
mismatches. Given a read with a phred-scaled probability q of being generated
from the mapped position, the new mapping quality is about sqrt((INT-
q)/INT)*INT. A zero value disables this functionality; if enabled, the
recommended value for BWA is 50. [0]
-d, --max-depth INT
At a position, read maximally INT reads per input file. Setting this limit
reduces the amount of memory and time needed to process regions with very high
coverage. Passing zero for this option sets it to the highest possible value,
effectively removing the depth limit. [8000]
Note that up to release 1.8, samtools would enforce a minimum value for this
option. This no longer happens and the limit is set exactly as specified.
-E, --redo-BAQ
Recalculate BAQ on the fly, ignore existing BQ tags. See BAQ below.
-f, --fasta-ref FILE
The faidx-indexed reference file in the FASTA format. The file can be optionally
compressed by bgzip. [null]
Supplying a reference file will enable base alignment quality calculation for
all reads aligned to a reference in the file. See BAQ below.
-G, --exclude-RG FILE
Exclude reads from read groups listed in FILE (one @RG-ID per line)
-l, --positions FILE
BED or position list file containing a list of regions or sites where pileup or
BCF should be generated. Position list files contain two columns (chromosome and
position) and start counting from 1. BED files contain at least 3 columns
(chromosome, start and end position) and are 0-based half-open.
While it is possible to mix both position-list and BED coordinates in the same
file, this is strongly ill advised due to the differing coordinate systems.
[null]
-q, --min-MQ INT
Minimum mapping quality for an alignment to be used [0]
-Q, --min-BQ INT
Minimum base quality for a base to be considered. [13]
Note base-quality 0 is used as a filtering mechanism for overlap removal which
marks bases as having quality zero and lets the base quality filter remove them.
Hence using --min-BQ 0 will make the overlapping bases reappear, albeit with
quality zero.
-r, --region STR
Only generate pileup in region. Requires the BAM files to be indexed. If used
in conjunction with -l then considers the intersection of the two requests. STR
[all sites]
-R, --ignore-RG
Ignore RG tags. Treat all reads in one BAM as one sample.
--rf, --incl-flags STR|INT
Required flags: include reads with any of the mask bits set [null]
--ff, --excl-flags STR|INT
Filter flags: skip reads with any of the mask bits set
[UNMAP,SECONDARY,QCFAIL,DUP]
-x, --ignore-overlaps-removal, --disable-overlap-removal
Overlap detection and removal is enabled by default. This option turns it off.
When enabled, where the ends of a read-pair overlap the overlapping region will
have one base selected and the duplicate base nullified by setting its phred
score to zero. It will then be discarded by the --min-BQ option unless this is
zero.
The quality values of the retained base within an overlap will be the summation
of the two bases if they agree, or 0.8 times the higher of the two bases if they
disagree, with the base nucleotide also being the higher confident call.
-X Include customized index file as a part of arguments. See EXAMPLES section for
sample of usage.
Output Options:
-o, --output FILE
Write pileup output to FILE, rather than the default of standard output.
-O, --output-BP
Output base positions on reads in orientation listed in the SAM file (left to
right).
--output-BP-5
Output base positions on reads in their original 5' to 3' orientation.
-s, --output-MQ
Output mapping qualities encoded as ASCII characters.
--output-QNAME
Output an extra column containing comma-separated read names. Equivalent to
--output-extra QNAME.
--output-extra STR
Output extra columns containing comma-separated values of read fields or read
tags. The names of the selected fields have to be provided as they are described
in the SAM Specification (pag. 6) and will be output by the mpileup command in
the same order as in the document (i.e. QNAME, FLAG, RNAME,...) The names are
case sensitive. Currently, only the following fields are supported:
QNAME, FLAG, RNAME, POS, MAPQ, RNEXT, PNEXT
Anything that is not on this list is treated as a potential tag, although only
two character tags are accepted. In the mpileup output, tag columns are
displayed in the order they were provided by the user in the command line.
Field and tag names have to be provided in a comma-separated string to the
mpileup command. Tags with type B (byte array) type are not supported. An
absent or unsupported tag will be listed as "*". E.g.
samtools mpileup --output-extra FLAG,QNAME,RG,NM in.bam
will display four extra columns in the mpileup output, the first being a list of
comma-separated read names, followed by a list of flag values, a list of RG tag
values and a list of NM tag values. Field values are always displayed before tag
values.
--output-sep CHAR
Specify a different separator character for tag value lists, when those values
might contain one or more commas (,), which is the default list separator. This
option only affects columns for two-letter tags like NM; standard fields like
FLAG or QNAME will always be separated by commas.
--output-empty CHAR
Specify a different 'no value' character for tag list entries corresponding to
reads that don't have a tag requested with the --output-extra option. The
default is *.
This option only applies to rows that have at least one read in the pileup, and
only to columns for two-letter tags. Columns for empty rows will always be
printed as *.
-M, --output-mods
Adds base modification markup into the sequence column. This uses the Mm and Ml
auxiliary tags (or their uppercase equivalents). Any base in the sequence
output may be followed by a series of strand code quality strings enclosed
within square brackets where strand is "+" or "-", code is a single character
(such as "m" or "h") or a ChEBI numeric in parentheses, and quality is an
optional numeric quality value. For example a "C" base with possible 5mC and
5hmC base modification may be reported as "C[+m179+h40]".
Quality values are from 0 to 255 inclusive, representing a linear scale of
probability 0.0 to 1.0 in 1/256ths increments. If quality values are absent (no
Ml tag) these are omitted, giving an example string of "C[+m+h]".
Note the base modifications may be identified on the reverse strand, either due
to the native ability for this detection by the sequencing instrument or by the
sequence subsequently being reverse complemented. This can lead to modification
codes, such as "m" meaning 5mC, being shown for their complementary bases, such
as "G[-m50]".
When --output-mods is selected base modifications can appear on any base in the
sequence output, including during insertions. This may make parsing the string
more complex, so also see the --no-output-ins-mods and --no-output-ins options
to simplify this process.
--no-output-ins
Do not output the inserted bases in the sequence column. Usually this is
reported as "+length sequence", but with this option it becomes simply
"+length". For example an insertion of AGT in a pileup column changes from
"CCC+3AGTGCC" to "CCC+3GCC".
Specifying this option twice also removes the "+length" portion, changing the
example above to "CCCGCC".
The purpose of this change is to simplify parsing using basic regular
expressions, which traditionally cannot perform counting operations. It is
particularly beneficial when used in conjunction with --output-mods as the
syntax of the inserted sequence is adjusted to also report possible base
modifications, but see also --no-output-ins-mods as an alternative.
--no-output-ins-mods
Outputs the inserted bases in the sequence, but excluding any base
modifications. This only affects output when --output-mods is also used.
--no-output-del
Do not output deleted reference bases in the sequence column. Normally this is
reported as "+length sequence", but with this option it becomes simply
"+length". For example an deletion of 3 unknown bases (due to no reference
being specified) would normally be seen in a column as e.g. "CCC-3NNNGCC", but
will be reported as "CCC-3GCC" with this option.
Specifying this option twice also removes the "-length" portion, changing the
example above to "CCCGCC".
The purpose of this change is to simplify parsing using basic regular
expressions, which traditionally cannot perform counting operations. See also
--no-output-ins.
--no-output-ends
Removes the “^” (with mapping quality) and “$” markup from the sequence column.
--reverse-del
Mark the deletions on the reverse strand with the character #, instead of the
usual *.
-a Output all positions, including those with zero depth.
-a -a, -aa
Output absolutely all positions, including unused reference sequences. Note
that when used in conjunction with a BED file the -a option may sometimes
operate as if -aa was specified if the reference sequence has coverage outside
of the region specified in the BED file.
BAQ (Base Alignment Quality)
BAQ is the Phred-scaled probability of a read base being misaligned. It greatly helps to
reduce false SNPs caused by misalignments. BAQ is calculated using the probabilistic
realignment method described in the paper “Improving SNP discovery by base alignment
quality”, Heng Li, Bioinformatics, Volume 27, Issue 8
<https://doi.org/10.1093/bioinformatics/btr076>
BAQ is turned on when a reference file is supplied using the -f option. To disable it,
use the -B option.
It is possible to store precalculated BAQ values in a SAM BQ:Z tag. Samtools mpileup will
use the precalculated values if it finds them. The -E option can be used to make it
ignore the contents of the BQ:Z tag and force it to recalculate the BAQ scores by making a
new alignment.
AUTHOR
Written by Heng Li from the Sanger Institute.
SEE ALSO
samtools(1), samtools-depth(1), samtools-sort(1), bcftools(1)
Samtools website: <http://www.htslib.org/>