APOB and familial hypercholesterolemia: In search of a potential mechanistic explanation of the observed hypercholesterolemia in the TMEM199 and CCDC115 deficient patients, we next studied apoB secretion in 2 complementary cell models: HepG2 cells in which these factors had been silenced and induced pluripotent stem cell (iPSC)-derived hepatocytes from patients with TMEM199 deficiency.