In this paper, we show that FD-ELP1 exon 20, which is the splicing event defective in Familial Dysautonomia, follows the kinetic model of co-transcriptional splicing and, like SMN2 exon 7, belongs to the type II alternative exons, being positively affected by the RNAPII elongation rate. This evidence concerns the gene SMN2 and Riley-Day syndrome.