Examples of cause by absence (c-a) are Anion Exchanger 1 in Spherocytosis [22]; Fumarate Hydratase in Fumarase Deficiency Disease (FHD, OMIM 606812); Securin in cancer [23]; Methyl-CpG-Binding Protein 2 in mental retardation (MR, OMIM 300055); or PKM in cardiovascular disease [24]. The gene discussed is MECP2; the disease is cancer.