A rare genetic disease (MIM:607803) conferring hypomagnesaemia, i.e., low Mg2+ level in the blood leading to tetany, seizure and cardiac arrhythmia, has been mapped to mutations in CNNM2, which is involved in Mg2+ excretion in kidney (Goytain and Quamme, 2005; Hirata et al., 2014). The gene discussed is CNNM2; the disease is cardiac rhythm disease.