Multiple pathways recruit and activate WASp for normal IS assembly. In normal lymphocytes, WASp is recruited to the IS following TCR ligation. (A) Zeta-associated protein 70 (Zap 70) has been shown to localize WASp, after CD3 stimulation, by 2 separate mechanisms.^25,57  The first is via EVH1 domain interaction with WIP and formation of a Zap 70-CrkL-WIP-WASp complex²⁷ ; the second is through direct interaction of the proline-rich region (PPR) of WASp with the SH3 domain of Nck.^25,28  The adaptor protein SLP76 is phosphorylated by Zap 70 (red hatched arrow) and binds both Nck and Vav-1 (a Cdc42-interacting guanosine diphosphate [GDP]–GTP exchange factor), thereby localizing both WASp and GTP-bound Cdc42 to the IS. (B) A Cdc42-independent route of WASp activation has recently been reported. Fyn phosphorylates WASp at Y291 (red hatched arrow) following TCR stimulation and is critical for multiple TCR-induced WASp effector functions.²⁹  WASp activity can be down-regulated by PST-PEST–mediated dephosphorylation (yellow hatched block), through CD2AP/PSTPIP1, ^26,29  indicating that the state of phosphorylation alone can regulate WASp activity.