Estrogen receptor alpha (ERalpha) is a ligand-inducible transcription factor that mediates the biological effects of estrogens and antiestrogens. Many point mutations in the human ERalpha gene have been reported to be associated with breast cancer, endometrial cancer, and psychiatric diseases. However, functional analyses for most mutants with amino acid changes are still lacking. In the present study, to investigate the effects of point mutations on the function, gel-shift assays and luciferase assays were performed for eight kinds of mutated ERalpha proteins, including a single nucleotide change of C207G (N69K), G478T (G160C), T887C (L296P), A908G (K303R), C926T (S309F), A1058T (E353V), A1186G (M396V), and G1231deletion (411fsX7). The mutated ERalpha expression plasmids were constructed by site-directed mutagenesis. With gel-shift assays using in vitro translated ERalpha proteins, binding to the consensus estrogen response element (ERE) was observed for the mutated ERalpha proteins except ERalpha (G160C) and ERalpha (411fsX7), the binding of which was comparable with that of the wild type. Western blot analyses showed that ERalpha (G160C) could not be efficiently translated with the in vitro transcription/translation system and that ERalpha (411fsX7) produced a truncated protein. To investigate the transactivation potency, wild-type or mutated ERalpha expression plasmids were co-transfected with pGL3-3EREc38 reporter plasmid into human breast adenocarcinoma MDA-MB-435 cells. The concentration-response curves (10pM-100nM E2) of the mutant ERalpha proteins except ERalpha (E353V) and ERalpha (411fsX7) were similar to that of wild-type ERalpha. However, at a low level of E2 (100pM), the mutants ERalpha (N69K), ERalpha (L296P), ERalpha (S309F), and ERalpha (M396V) showed a significant decrease of transactivation compared with that of the wild-type ERalpha. The mutants ERalpha (E353V) and ERalpha (411fsX7) did not show responsiveness to E2 and antiestrogens, 4-hydroxytamoxifen (4OHT) and ICI 182,780. The mutant ERalpha (S309F) showed decreased responsiveness for the antiestrogenicity of 4OHT. In conclusion, we found that some of the naturally occurring human ERalpha mutants with amino acid changes may have an altered responsiveness to estrogen and antiestrogens.