Transient receptor potential (TRP) cation-selective channels are an emerging class of proteins that are involved in a variety of important biological functions including pain transduction, thermosensation, mechanoregulation, and vasorelaxation. Utilizing a bioinformatics approach, we have identified the full-length human TRPM3 (hTRPM3) as a member of the TRP family. Following the identification of the founding member of this family, dTRP, which is from a Drosophila mutant with abnormal visual signal transduction (2), mammalian homologues have been cloned and all of them contain a six-transmembrane domain followed by a TRP motif (XWKFXR). Based on homology, they are divided into three subfamilies: TRPC (canonical), TRPV (vanilloid), and TRPM (melastatin) (3). Members of the TRPM subfamily have unusually long cytoplasmic tails at both ends of the channel domain, and some of the family members have an enzyme domain in the C-terminal region. Despite their similarities of structure, TRPMs have different ion-conductive properties, activation mechanisms, and putative biological functions. TRPM1 is down-regulated in metastatic melanomas (4). TRPM2 is a Ca 2ϩ -permeable channel that contains an ADP-ribose pyrophosphatase domain and can be activated by ADP-ribose, NAD (5, 6), and changes in redox status (7). The TRPM2 gene is mapped to the chromosome region linked to bipolar affective disorder, nonsyndromic hereditary deafness, Knobloch syndrome, and holosencephaly (8). Two splice variants of TRPM4 have been described. TRPM4a is predominantly a Ca 2ϩ -permeable channel (9); whereas TRPM4b conducts monovalent cations upon activation by changes in intracellular Ca 2ϩ (10). TRPM5 is associated with Beckwith-Wiedemann syndrome and a predisposition to neoplasias (11). TRPM7, another bifunctional protein, has kinase activity in addition to its ion channel activity. TRPM7 is regulated by Mg 2ϩ -ATP and/or inositol 1,4,5-disphosphate and is required for cell viability (12-14). TRPM8 is up-regulated in prostate cancer and other malignancies (15). Recently, it has been shown to be a receptor that senses cold stimuli (16,17).Using a bioinformatics approach, we have identified a member of the human TRPM subfamily that we have called hTRPM3, consistent with the unified TRP nomenclature (3). hTRPM3 contains long N and C termini, although it does not contain any additional enzymatic features. hTRPM3 mRNA is expressed primarily in kidney with lower levels in brain, testis, and spinal cord. When expressed in HEK 293 cells, hTRPM3 is co-localized with the plasma membrane and is capable of mediating Ca 2ϩ entry. This hTRPM3-mediated Ca 2ϩ conductance * The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.The nucleotide sequence (s)