A 4-month-old infant presents with macrocephaly, hepatosplenomegaly, and severe anemia. The parents report he seems less responsive to sounds and visual stimuli than his older sibling was at this age. Radiographs reveal diffuse bone sclerosis with a “bone-within-bone” appearance. Molecular testing confirms autosomal recessive osteopetrosis. Which of the following genetic defects would prevent successful treatment with hematopoietic stem cell transplantation (HSCT)?