①adapter trimming
The sequencing libraries usually contain large amount of adapter sequencing, which can not be aligned to genome. Therefore, it is required to process the raw data before large-scale sequence alignment, such as adapter trimming.

②mapping to genome
Since the origin of species and sample types DNA sample, we need to select the appropriate genome sequence alignment; the most common include Ensembl human genome assembly and UCSF Homo sapiens GRCh38 genome.