IGHMBP2 and Charcot-Marie-Tooth disease axonal type 2S: On the other hand, mutations in IGHMBP2 lead to a large disease spectrum with no clear genotype–phenotype correlation, which includes Spinal Muscular Atrophy with Muscular Distress type 1 (SMARD1), an extremely rare form of SMA, and Charcot–Marie–Tooth 2S (CMT2S).