['Homozygous familial hypercholesterolemia is a rare clinical phenotype with a variable expression, which is characterized by extremely elevated plasma low-density lipoprotein (LDL), tendon and skin xanthomas, and a progressive atherosclerosis. In 95% of patients, homozygous familial hypercholesterolemia is due to mutations in low-density lipoprotein receptor (LDLR) gene, which abolish (receptor-negative) or greatly reduce (receptor-defective) LDLR function.', 'The objective of the study was the molecular and phenotypic characterization of 4 siblings with severe hypercholesterolemia.', 'The major LDL-related genes (LDLR, APOB, PCSK9, ANGPTL3, APOE, and APOC3) were sequenced. LDLR messenger RNA, isolated from leukocytes, was reverse transcribed and sequenced.', 'The index cases were 24-year-old identical twin sisters with long-standing tendon xanthomas and high low-density lipoprotein cholesterol (LDL-C ∼10\xa0mmol/L) but no coronary heart disease. They were carriers of 2 LDLR mutations: (1) a previously reported mutation [p.(G335S)] inherited from the mother who had LDL-C level within normal range; (2) a novel 24\xa0bp deletion in exon 8/intron 8 junction inherited from the hypercholesterolemic (LDL-C 6.1\xa0mmol/L) father. The deletion allele encodes an messenger RNA with a partial deletion of exon 8, whose translation product has an in-frame deletion of 17 amino acids [p.(Glu380_Gly396del)]. Family screening revealed that the 2 siblings of the twin sisters were also compound heterozygotes but had much lower LDL-C levels (8.2 and 7.1\xa0mmol/L). The sequence of potential modifying genes showed that the 2 siblings and the mother of the twin sisters were heterozygous for a rare missense variant of apoB [p.(S2429T)], which might have an LDL-lowering effect.'] Do phenotypic variability in 4 homozygous familial hypercholesterolemia siblings compound heterozygous for LDLR mutations?