HCFC1 and methylmalonic aciduria and homocystinuria type cblC: Patients with mutations in the Host Cell Factor 1 (HCFC1) locus (MIM 309541), located on the X chromosome, have a phenocopy of cblC disease [11] since HCFC1 encodes a transcriptional co-regulator of MMACHC. Recently additional defects have been described that affect transcription factors involved in the regulation of Cbl pathway: ZNF143 [12] and THAP11 [13].