T-cell activation requires 2 signals: recognition of antigen by the T-cell receptor (see TCR; MIM 186880) and a costimulatory signal provided primarily by CD28 (MIM 186760) in naive T cells. HSH2 is a target of both of these signaling pathways (Greene et al., 2003 [PubMed 12960172]).[supplied by OMIM, Mar 2008].