5 months old previously healthy male is admitted to the PICU with severe macrocytic anemia. His father and his father's sister also have macrocytic anemia. His father said that his anemia responded to Vit B12 supplementation though Vit B12 value was normal. CBC: WBC 5.3 K/ul L, Hb 2.1g/dL L, Hct 5.9% L, Plt 323, MCV 110 H, MCH 39 H, RDW 19 H, Reticulocyte 13K/ul, L.