Forms of MmD due to recessive mutations in the selenoprotein N (SEPN1) gene are probably rarer than those due to mutations in the skeletal muscle ryanodine receptor (RYR1) gene (personal observation), as mutations in the latter gene appear to be the most common genetic cause of a wide variety of congenital myopathies including central core disease (CCD) [5], certain forms of centronuclear myopathy (CNM) [6] and specific subgroups of MmD. The gene discussed is SELENON; the disease is congenital myopathy with cores.