Cholesteryl ester transfer protein (CETP) deficiency is one of the most important and common causes of hyperalphalipoproteinemia (HALP) in the Japanese. CETP deficiency is thought to be a state of impaired reverse cholesterol transport, which may possibly lead to the development of atherosclerotic cardiovascular disease despite high HDL-cholesterol (HDL-C) levels. Thus, it is important to investigate whether HALP is caused by CETP deficiency. In the present study, we identified two novel missense mutations in the CETP gene among 196 subjects with a marked HALP (HDL-C ജ 2.59 mmol/l ؍ 100 mg/dl). The two missense mutations, L151P (CTC → CCC in exon 5) and R282C (CGC → TGC in exon 9), were found in compound heterozygous subjects with D442G mutation, whose plasma CETP levels were significantly lower when compared with those in D442G heterozygous subjects. In COS-7 cells expressing the wild type and mutant CETP, these two mutant CETP showed a marked reduction in the secretion of CETP protein into media (0% and 39% of wild type for L151P and R282C, respectively). These results suggested that two novel missense mutations cause the decreased secretion of CETP protein into circulation leading to HALP. By using the Invader ® assay for seven mutations, including two novel mutations of the CETP gene, we investigated their frequency among 466 unrelated subjects with HALP (HDL-C ജ 2.07 mmol/l ؍ 80 mg/dl). Two novel mutations were rare, but L151P mutation was found in unrelated subjects with a marked HALP.Furthermore, we demonstrated that CETP deficiency contributes to 61.7% and 31.4% of marked HALP and moderate HALP in the Japanese, respectively. Reverse cholesterol transport (RCT) is one of the major protective systems against atherosclerosis, in which small HDL or free apolipoprotein (apo)A-I removes cholesterol from the peripheral cells and delivers it to the liver (1, 2). We have investigated the molecular mechanisms of RCT by analyzing the pathophysiology of subjects with abnormal HDL metabolism and clarified the physiological significance of plasma cholesteryl ester transfer protein (CETP) and hepatic triglyceride lipase in human RCT (3,4). In this system, CETP is known to facilitate the transfer of cholesteryl ester (CE) from HDL to apoB-containing lipoproteins, and is one of the major determinants of plasma HDL-cholesterol (HDL-C) levels in humans (5)(6)(7)(8). This is true especially in the hypertriglyceridemic subjects (9). Furthermore, CETP affects not only plasma HDL-C levels but also the amount of small HDL particles, which may be more important for RCT (8,10).Genetic CETP deficiency is the most important and frequent cause of hyperalphalipoproteinemia (HALP) in the Japanese (11-21). Seven mutations have been demonstrated to cause HALP, including two common mutations: a G-to-A substitution at the 5 Ј splicing donor site of the intron 14 (Int14 ϩ 1 G → A) and a missense mutation of exon Abbreviations: BPI, bactericidal permeability increasing protein; CE, cholesteryl ester; CETP, cholesteryl ester tra...