DBP is the major serum transport protein for the vitamin D sterols2, binds and sequesters monomers of actin with high affinity3, 4, and has been identified on the surface of a variety of cell types including B-lymphocytes5, subpopulations of T-lymphocytes6, and the cytotrophoblasts of the placenta7. DBP is a secreted protein and would therefore be predicted to have a signal sequence. The amino-terminus of the mature serum protein was established by aligning the sequence predicted by the cDNA to a sequenced amino-terminal hDBP peptide8.