These mutations result in a decreased production of cofactors for methylmalonyl-CoA mutase (adenosylcobalamin) and methionine synthase (MTR; methylcobalamin) and, in turn, in elevated Hcy levels in the cerebrospinal fluid that correlate with unilateral SNHL (Harding et al., 2003; Tsai et al., 2007; Carrillo-Carrasco et al., 2012). The gene discussed is MTR; the disease is sensorineural hearing loss disorder.