The 335-amino acid type II isoform lacks the UDPGA-binding domain and the transmembrane region. The 412-amino acid type III isoform retains the transmembrane region at its C-terminal end but lacks the substrate-binding domain. All 3 types possess only 1 N-linked glycosylation site. Western blot analysis showed expression of 52-, 35-, and 42-kD proteins, corresponding to the type I, II, and III UGT2B28 proteins, respectively. Northern blot analysis revealed UGT2B28 type I, II, and III transcripts of 1.6, 1.4, and 1.3 kb, respectively, expressed in a kidney cell line. RT-PCR analysis detected all 3 forms in liver and mammary gland. Type III was also expressed in several other tissues and breast cancer cell lines. Functional analysis indicated glucuronidating capacity with steroid substrates for UGT2B28 type I, but not for the type II or III forms.