MBNL1 and mitral valve prolapse: In addition, in the nucleotide repeat expansion disorder, myotonic dystrophy, reduced MBNL1 splicing activity (due to binding of MBNL1 protein to the expansion RNA) is thought to play a major role in determining the disease phenotype, which includes several cardiovascular abnormalities (conduction defects, arrhythmias, mitral valve prolapse) [43, 44].