The C-terminal CAAX motif of fas proteins undergoes a triplet of posttranslational modifications that are required for membrane association. The CAAX motif lies immediately C-terminal to the hypervariable domain, a region of 20 amino acids that distinguishes the ras proteins from each other. The hypervarlable domains of p21 ur, p21 un*, and p21K-‘**(*) contain she8 for palmltoylation, which we now show must combine with the CAAX motif to target specific plasma membrane localization. Wlthln the hypervariable domain of~ 21~-~**@), which Is not palmitoylated, we have identified a navel plasma membrane targeting slgnal consist-Ing of a polybaslc domain that also acts in comblnstion with the CAAX motif. One function of the hypervariable domains of~ 21” s is therefore to provide different signals for plasma membrane localization.