Mechanistic insights into human pre-mRNA splicing of human ultra-short introns: Potential unusual mechanism identifies G-rich introns

► We identified and characterized ultra-short introns (<65-nt) of human pre-mRNAs. ► The 56-, 49-, and 43-nt introns were found in the HNRNPH1, NDOR1, and ESRP2 genes. ► SF3b (U2 snRNP component) is involved in splicing of these 3 ultra-short introns. ► Splicing of the 56-nt intron requires the U1, U2, and U4 snRNPs. ► Splicing of the 49- and 43-nt introns requires G-rich intronic splicing enhancers.