FCS is: (1) very rare in the population, with revised estimated prevalence of ~1 in 200,000–300,000; (2) quite rare among patients with severe hypertriglyceridemia, making up perhaps 1% of this cohort; (3) molecularly heterogeneous, caused by rare biallelic variants in five canonical genes, although biallelic LPL variants underlie the large majority of cases; (4) relatively homogeneous phenotypically across various molecular etiologies; and (5) differentiated from MCM by younger age of onset, absence of secondary factors, low apo B level and at least 5-fold greater risk of acute pancreatitis. The gene discussed is APOB; the disease is acute pancreatitis.