Alternative splicing of exons forming CELF1 5′UTRs from P2 was also disrupted in DM1; in mRNAs from both promoters, ex5 (containing an alternative AUG codon) was mostly excluded, and the expression of shorter isoforms (mainly isoforms i1-P1, i1-P2 and i2-P2) was increased significantly (Fig. 1c). Here, CELF1 is linked to myotonic dystrophy type 1.