The 'CABIT' domain (for 'cysteine-containing, all- in Themis') is found in a newly identified gene family that has three mammalian homologs (Themis, Icb1 and 9130404H23Rik) that encode proteins with two CABIT domains and a highly conserved proline-rich region. In contrast, Fam59A, Fam59B and related proteins from mammals to cnidarians, including the insect Serrano proteins, have a single copy of the CABIT domain, a proline-rich region and often a C-terminal SAM (sterile-motif) domain. Multiple-sequence alignment has predicted that the CABIT domain adopts an all-strand structure with at least 12 strands, ie a dyad of six-stranded beta-barrel units. The CABIT domain contains a nearly absolutely conserved cysteine residue which is likely to be central to its function. CABIT domain proteins function downstream of tyrosine kinase signalling and interact with GRB2.