MBNL1 and cardiac arrhythmia: In urine cells, splicing patterns of individual transcripts showed no significant difference based on age of symptom onset, heel walking, myotonia, or cardiac arrhythmia, although ability to heel walk showed a non-significant trend for a difference of INSR, MBNL1, and MAP3K4 splicing (Supplementary Figs. 9, 10).