Themis is a member of a new metazoan gene family and is required for completion of thymocyte positive selection

T-cell receptor signaling in CD4(+)CD8(+) double positive thymocytes determines cell survival and lineage commitment, but the genetic and molecular basis of this process is poorly defined. To address this issue, we used ethylnitrosourea mutagenesis to identify a novel T-lineage-specific gene, Themis, which is critical for the completion of positive selection. Themis contains a tandem repeat of a unique globular domain (CABIT), which includes a cysteine motif that defines a family of 5 uncharacterized vertebrate proteins with orthologs in most animal species. Themis-deficient thymocytes showed no major impairment in early T-cell receptor signaling, but exhibited altered expression of cell cycle and survival genes before and during positive selection. These data suggest a unique role for Themis in sustaining positive selection.