1. Diagnosis of GM2 gangliosidosis, confirmed by demonstration of profound deficiency of -hexosaminidase A or A & B in peripheral blood leukocytes or in cultured skin fibroblasts, within the previous 1 year in non-bone marrow transplant recipients who are < 2 years of age, or prior to stem cell transplant in stably engrafted transplant patients who are < 5 years of age.