- CD28 and the tyrosine kinase lck stimulate mitogen-activated protein kinase activity in T cells via inhibition of the small G protein Rap1. Carey, K.D., Dillon, T.J., Schmitt, J.M., Baird, A.M., Holdorf, A.D., Straus, D.B., Shaw, A.S., Stork, P.J. Mol. Cell. Biol. (2000)