We previously reported that ATM heterozygous mutation in ApoE−/− mice resulted in an overaccumulation of plasma ApoB48-containing lipoproteins and severe hypercholesterolemia occurred only with a combination of a heterozygous ATM mutation and a null ApoE mutation, but not wih the heterozygous ATM mutation alone or with the combined heterozygous ATM and null LDL receptor mutations (7). The gene discussed is APOE; the disease is familial hypercholesterolemia.