当我们使用bwa或者bowtie map reads之后，当我们察看日志文件，我们会看到诸如：

 20164115 reads; of these:   20164115 (100.00%) were unpaired; of these:     255188 (1.27%) aligned 0 times     14142345 (70.14%) aligned exactly 1 time     5766582 (28.60%) aligned >1 times 98.73% overall alignment rate 

这样的报告。很多人会告诉我们，对于下游分析，需要去除map至多个位点的reads