GU–AG introns are found in:
Major class introns removed by spliceosome share highly conserved terminal dinucleotides, GU at 5' end and AG at 3' end, recognized by U1 and U2 snRNPs respectively. These U2-type GU-AG introns constitute vast majority, over 99% of introns in eukaryotic nuclear pre-mRNA, distinguished by consensus sequences including branch point adenine. They reside in protein-coding genes transcribed by RNA polymerase II in nucleus, removed co-transcriptionally. Minor U12-dependent AU-AC introns represent rare variant. GU-AG is absent from tRNA introns or group I/II self-splicing introns found in organelles.
Ref: Steitz and Moore, RNA Splicing, GU-AG Introns in Eukaryotic Nuclear pre-mRNA, NCBI Bookshelf