Post-transcriptional processing of nearly all mRNA primary transcripts occurs in eukaryotic cells. Introns are removed and exons are joined in the process of RNA splicing. An intron usually contains a clear splicing signal, which are short sequences framing the intron as well as an internal sequence called the branch site. Sometimes a regulatory protein may mask the splicing signal, resulting in alternative processing. RNA splicing occurs by two transesterification reactions, not hydrolysis followed by ligation as might be expected.