Endogenous NF1 exon 23a inclusion patterns and UG-rich upstream intronic sequence. (A) A representation of endogenous NF1 exon 23a inclusion patterns. Exon 23a is predominantly included in most tissues, while it is predominantly skipped in neurons. Arrows represent RT-PCR oligonucleotides designed to analyze NF1 exon 23a splicing patterns. (B) The intronic region upstream of NF1 exon 23a is UG-rich and harbors two potential Muscleblind-like (MBNL) protein binding motifs (UGCUGU) which are underlined.