The hereditary stomatocytoses (HSt) are caused by defects of membrane cation permeability and cell volume regulation; they are divided in two different entities: dehydrated hereditary stomatocytosis (DHSt) due to mutations in PIEZO1 and KCNN4 genes, and overhydrated hereditary stomatocytosis (OHS) associated with mutations in RhAG gene; additional more rare forms of stomatocytosis include defects in ABCB6 (pseudohyperkaliemia) or GLUT1 (cryohydrocytosis with neurological impairment) (Glogowska and Gallagher, 2015; Badens and Guizouarn, 2016). The gene discussed is RHAG; the disease is hereditary stomatocytosis.