We have introduced eleven point mutations into the human vitamin D receptor by site-directed mutagenesis in order to identify some of the amino acid residues that are important for ligand binding. The amino acid residues Ser225, His229, Asp232, Val234, Ser235, Tyr236, Ser237, Lys240, Ile242, Lys246 (helix 3), and Ser275 (helix 5) of the human vitamin D receptor were substituted by alanine. We report here that His229, Asp232, and Ser237 have an important role in the binding of calcitriol. In addition, the amino acid residues Tyr236 and Ser275 also seem to participate in the ligand binding process.