RESUMEN
Menkes disease (MD) is a rare X-linked recessive disorder caused by mutations in the ATP7A gene. This neurodegenerative disorder typically affects males and is characterized by impaired copper distribution and the malfunction of several copper-dependent enzymes. We report clinically discordant female monozygotic twins (MZT) with a heterozygous ATP7A mutation. One twin girl is healthy at the current age of 4 years, whereas the other twin girl developed classical MD, showed disease stabilization under copper histidine treatment but died at the age of 3 years. Presumably, the affected girl developed MD due to skewed X inactivation, although this could not be demonstrated in two tissues (blood, buccal mucosa). This case is a rare example of an affected girl with MD and shows the possibility of a discordant phenotype in MZT girls. As speculated in other X-linked diseases, the process of monozygotic twinning may be associated with skewed X inactivation leading to a discordant phenotype.