IGHMBP2 and Spinal muscular atrophy with respiratory distress type 1: An extremely rare form of SMA is Spinal Muscular Atrophy with Respiratory Distress type 1 or distal Spinal Muscular Atrophy type 1 (SMARD1/DSMA1), which is caused by a mutation in the IGHMBP2 gene, located on chromosome 11.