Cerebellar beta 2 chimaerin , a GTPase activating protein for p 21 ras related rac is specifically expressed in granule cells and has a unique N terminal SH 2 domain . beta Chimaerin , a 30 kDa GTPase activating protein ( GAP ) for the p 21 Ras related Rac , is expressed specifically in late stage spermatocytes ( Leung , T . , How , B . ^^^