IGHMBP2 and Charcot-Marie-Tooth disease axonal type 2S: Homozygous or compound heterozygous mutations in immunoglobulin mu DNA binding protein 2 (IGHMBP2) result in 2 distinct diseases: SMA with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S).