Familial combined hyperlipidemia (FCHL) is characterized by fluctuations in serum lipid concentrations and may present as mixed hyperlipidemia, isolated hypercholesterolemia, hypertriglyceridemia, or as a normal serum lipid profile in combination with abnormally elevated levels of apolipoprotein B (APOB; 107730). Patients with FCHL are at increased risk of cardiovascular disease and mortality and have a high frequency of comorbidity with other metabolic conditions such as type 2 diabetes, nonalcoholic fatty liver disease, steatohepatitis, and the metabolic syndrome (summary by Bello-Chavolla et al., 2018). Goldstein et al. (1973) gave the designation 'familial combined hyperlipidemia' to the most common genetic form of hyperlipidemia identified in a study of survivors of myocardial infarction. Affected persons characteristically showed elevation of both cholesterol and triglycerides in the blood. The combined disorder was shown to be distinct from familial hypercholesterolemia (143890) and from familial hypertriglyceridemia (145750) for the following reasons: (1) lipid distributions in relatives were unique; (2) unlike familial hypercholesterolemia, children of affected persons did not express hypercholesterolemia; and (3) informative matings suggested that variable expression of a single gene rather than segregation for 2 separate genes was responsible. This disorder leads to elevated levels of VLDL, LDL, or both in plasma. From time to time the pattern can change in a given person. Unlike familial hypercholesterolemia, hyperlipidemia appears in only 10 to 20% of patients in childhood, usually in the form of hypertriglyceridemia. Xanthomas are rare. Increased production of VLDL may be a common underlying metabolic characteristic in this disorder, which may be heterogeneous. The disorder may be 5 times as frequent as familial hypercholesterolemia, occurring in 1% of the U.S. population. Genetic Heterogeneity of Susceptibility to Familial Combined Hyperlipidemia Also see FCHL1 (602491), associated with variation in the USF1 gene (191523) on chromosome 1q23, and FCHL2 (604499), mapped to chromosome 11.