http://blog.sina.com.cn/s/blog_5d5f892a0102we4t.html
http://sihua.us/samtools.htm
Pileup Format
Pileup format is first used by Tony Cox and Zemin Ning at the Sanger Institute. It desribes the base-pair information at each chromosomal position. This format facilitates SNP/indel calling and brief alignment viewing by eyes.
The pileup format has several variants. The default output by SAMtools looks like this:
seq1 272 T 24 ,.$.....,,.,.,...,,,.,..^+. <<<+;<<<<<<<<<<<=<;<;7<& seq1 273 T 23 ,.....,,.,.,...,,,.,..A <<<;<<<<<<<<<3<=<<<;<<+ seq1 274 T 23 ,.$....,,.,.,...,,,.,... 7<7;<;<<<<<<<<<=<;<;<<6 seq1 275 A 23 ,$....,,.,.,...,,,.,...^l. <+;9*<<<<<<<<<=<<:;<<<< seq1 276 G 22 ...T,,.,.,...,,,.,.... 33;+<<7=7<<7<&<<1;<<6< seq1 277 T 22 ....,,.,.,.C.,,,.,..G. +7<;<<<<<<<&<=<<:;<<&< seq1 278 G 23 ....,,.,.,...,,,.,....^k. %38*<<;<7<<7<=<<<;<<<<< seq1 279 C 23 A..T,,.,.,...,,,.,..... ;75&<<<<<<<<<=<<<9<<:<<where each line consists of chromosome, 1-based coordinate, reference base, the number of reads covering the site, read bases and base qualities. At the read base column, a dot stands for a match to the reference base on the forward strand, a comma for a match on the reverse strand, `ACGTN' for a mismatch on the forward strand and `acgtn' for a mismatch on the reverse strand. A pattern `+[0-9]+[ACGTNacgtn]+' indicates there is an insertion between this reference position and the next reference position. The length of the insertion is given by the integer in the pattern, followed by the inserted sequence. Here is an example of 2bp insertions on three reads:
seq2 156 A 11 .$......+2AG.+2AG.+2AGGG <975;:<<<<<Similarly, a pattern `-[0-9]+[ACGTNacgtn]+' represents a deletion from the reference. Here is an exmaple of a 4bp deletions from the reference, supported by two reads:
seq3 200 A 20 ,,,,,..,.-4CACC.-4CACC....,.,,.^~. ==<<<<<<<<<<<::<;2<<Also at the read base column, a symbol `^' marks the start of a read segment which is a contiguous subsequence on the read separated by `N/S/H' CIGAR operations. The ASCII of thecharacter following `^' minus 33 gives the mapping quality. A symbol `$' marks the end of a read segment. Start and end markers of a read are largely inspired by Phil Green's CALF format. These markers make it possible to reconstruct the read sequences from pileup.
SAMtools can optionally append mapping qualities to each line of the output. This makes the output much larger, but is necessary when a subset of sites are selected.
mpileup生成的结果包含6行：参考序列名；位置；参考碱基；比对上的reads数；比对情况；比对上的碱基的质量。其中第5列比较复杂,解释如下：
1 ‘.’代表与参考序列正链匹配。
2 ‘,’代表与参考序列负链匹配。
3 ‘ATCGN’代表在正链上的不匹配。
4 ‘atcgn’代表在负链上的不匹配。
5 ‘*’代表模糊碱基
6 ‘^’代表匹配的碱基是一个read的开始；’^'后面紧跟的ascii码减去33代表比对质量；这两个符号修饰的是后面的碱基，其后紧跟的碱基(.,ATCGatcgNn)代表该read的第一个碱基。
7 ‘$’代表一个read的结束，该符号修饰的是其前面的碱基。
8 正则式’+[0-9]+[ACGTNacgtn]+’代表在该位点后插入的碱基；比如上例中在scaffold_1的2847后插入了9个长度的碱基acggtgaag。表明此处极可能是indel。
9 正则式’-[0-9]+[ACGTNacgtn]+’代表在该位点后缺失的碱基；