名词：
- gene model (e.g., exon, intron, splice site, etc.)
predicts whether an amino acid substitution affects protein function
函数
mcols() 把 GRanges 转换为 DataFrame
> class(all)
[1] "GRanges"
attr(,"package")
[1] "GenomicRanges"
> class(mcols(all))
[1] "DataFrame"
attr(,"package")
[1] "S4Vectors"
