Nested introns in an intron: Evidence of multi-step splicing in a large intron of the human dystrophin pre-mRNA

► Huge intron 7 spliced from endogenous dystrophin pre-mRNA was analyzed by RT-PCR. ► The excised lariat intron 7 had a closed loop but it contained gaps in its sequence. ► Two presumed lariats generated from nested introns within intron 7 were detected. ► These RNA products were generated concomitantly with dystrophin pre-mRNA splicing. ► Proposed model: nested introns are spliced before ultimate splicing in huge intron.