chromosome Xp11.23-p11.22 duplication syndrome (MONDO_0010428, a Monarch Disease Ontology id) is a form of methylmalonic acidemia with homocystinuria (see this term), an inborn error of vitamin B12 (cobalamin) metabolism characterized by megaloblastic anemia, lethargy, failure to thrive, developmental delay, intellectual deficit and seizures. Also known as: Xp11.22-p11.23 Microduplication, chromosome xp11.23-p11.22 duplication syndrome, X-linked dominant, microduplication Xp11.22-p11.23 syndrome, trisomy Xp11.22-p11.23.