High density lipoproteins (HDLs) play a critical role in cholesterol metabolism. HDL plasma concentrations are inversely correlated with risk for atherosclerosis. SR-BI and SR-BII (previously known as SR-BI.2) are the alternatively spliced products of a single gene. SR-BII differs from SR-BI in that the encoded c-terminal cytoplasmic domain is almost completely different. SR-BII binds HDLs mediating selective uptake of HDL cholesteryl ester; with an approximately 4-fold lower efficiency than SR-BI. Nuclease protection assays show SR-BII to be abundant in mouse tissues expressing SR-BI. SR-BII expression found in liver, adrenal glands, and testes. The role of SR-BII is not completely clear. Research suggests that it may be a functional HDL receptor. SR-BII mRNA results from the alternative splicing of SR-BI precursor transcripts with the SR-BII isoform mediating selective transfer of lipid between HDL and cells. The relative expression and functional activities of these two isoforms create a potential means of regulating selective lipid transfer between HDL and cells (1-6).