An extracellular spherical HDL particle binds to the plasma membrance-associated SR-BI receptor with high affinity (Murao et al. 1997). In the body SR-BI receptors are abundant on the surfaces of steroidogenic cells in the adrenal glands and gonads, and on hepatocytes. SR-BI thus appears to play a central role in cholesterol uptake for steroid hormone synthesis and for bile acid synthesis and cholesterol excretion (Rigotti et al. 2003; Silver and Tall 2001).