What it does

Generate pileup for one or multiple BAM files. Alignment records are grouped by sample (SM) identifiers in @RG header lines. If sample identifiers are absent, each input file is regarded as one sample.

Generation of VCF and BCF output, is deprecated and not available in the Galaxy tool. Please use bcftools mpileup for this instead.

In the pileup format (without -u or -g), each line represents a genomic position, consisting of chromosome name, 1-based coordinate, reference base, the number of reads covering the site, read bases, base qualities and alignment mapping qualities. Information on match, mismatch, indel, strand, mapping quality and start and end of a read are all encoded at the read base column. At this column, a dot stands for a match to the reference base on the forward strand, a comma for a match on the reverse strand, a '>' or '<' for a reference skip, 'ACGTN' for a mismatch on the forward strand and 'acgtn' for a mismatch on the reverse strand. A pattern '\+[0-9]+[ACGTNacgtn]+' indicates there is an insertion between this reference position and the next reference position. The length of the insertion is given by the integer in the pattern, followed by the inserted sequence. Similarly, a pattern '-[0-9]+[ACGTNacgtn]+' represents a deletion from the reference. The deleted bases will be presented as '*' in the following lines. Also at the read base column, a symbol '^' marks the start of a read. The ASCII of the character following '^' minus 33 gives the mapping quality. A symbol '$' marks the end of a read segment.

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.

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 pre-calculated 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.