Antigen binding by the T-cell receptor (TCR) is one of the critical first steps in the immune response, triggering a cascade of signaling pathways that ultimately lead to T-cell activation.   Screening a yeast two-hybrid screen of a human T-cell cDNA library with calcineurin, a protein phosphatase involved in multiple signaling pathways including T-cell activation, resulted in the identification of Carabin, a member of the TBC1 domain family of proteins, as a calcineurin-binding protein.  Unlike other members of the TBC1 domain protein family which are thought to have a role in regulating cell growth and differentiation, further experiments demonstrated that Carabin is part of a negative regulatory loop for the intracellular TCR signaling pathway as well as an inhibitor of the Ras signaling pathway, suggesting that Carabin may also mediate crosstalk between calcineurin and Ras.  Carabin antibody does not recognize TBC1D10A or TBC1D10B.  Carabin is known to exist in multiple isoforms.