The family of fork head or winged-helix proteins coniprises a large nunil)er of transcription factors, defined by a conserved I)NA binding douiain, hi the last, few years, over 80 nienibers of this family have been identified in species ranging from yeast to inan. Individual members perfornl various biological functions: their expression patterns are usually temporally and spatially restricted and some of these genes appear to play" a central role in embryonic development. We describe the cloning of Trident, a novel member of the fork head/wingedhelix family from nlurine thymus. In the mouse embryo the gene was expressed in all tissues, whereas in adult mice expression was only detected in the tliyruns. l)nring T (:ell differentiation, expression peaked in the actively dividing immature single positive cells. In peripheral blood lymphocytes expression of Trident mRNA was absent, but could be induced upon stimulation with mitogens in vitro. Further analysis revealed that Trident expression strictly' correlated with cell cycling, independent of ell type. Timing of 3tt-thymidine incorporation showed that mRNA and protein expression were st rongly upregulated upon entry' into the S phase of the cell cycle. PCR-mediated selection of optimal binding sites yielded a consensus motif resembling that of other family members, ltowever, selected binding sequences consisted of multiple motifs with fixed spacing and orientation, indicative of di- and trimerization of the protein. These results identify" Trident as a transcription factor, which is likely involved in cell-cycle specific gelie regulation.