TCN2 and myelodysplastic syndrome: Patient Pat-1088 who presented with pancytopenia, myelodysplastic syndrome on bone marrow biopsy, 5% blasts in the peripheral blood, mild developmental delay, abnormal skin pigmentation, and intestinal obstructions had a dramatic response to hydroxycobalamin injections after he was found to have a cobalamin defect caused by a homozygous TCN2 variant.