. Placental alkaline phosphatase (PLAP) is anchored in the plasma membrane by a phosphatidylinositol-glycan moiety (PI-glycan) . PIglycan is added posttranslationally to the nascent peptide chain after the removal of 29 amino acids from the COOH-terminus . The contribution of selected COOH-terminal amino acids to the signal for PIglycan addition was tested by creating a fusion protein with the COOH-terminus of PLAP and a secreted protein and by mutagenesis of specific PLAP 000H-terminal amino acids . The cDNA encoding the COOH-terminus of PLAP was fused in frame to the cDNA for human clotting Factor X and expressed in transfected COS-1 cells. Fusion proteins containing 32 amino acids of the PLAP COON-terminus were modified by PI-glycan addition. Thus, the signal for Pl-glycan modification must reside in these amino acids . Next, the region between the hydrophobic domain and the cleavage site was examined for additional number of proteins are anchored in the plasma membrane by a COOH-terminal glycosyl phosphatidylinositol moiety (PI-g1Ycan) (Ferguson and Williams, 1988) . cDNA sequences for Pl-glycan-anchored proteins predict a typical N112-terminal signal peptide that directs the protein to the ER and a 20-30 amino acid COOH-terminal sequence that is absent in the mature PI-glycan-anchored membrane proteins . The PI-glycan attachment occurs in the ER shortly after the peptide chain is synthesized, presumably with the removal of a COOH-terminal peptide (Doering et al., 1990) . The modified protein is then transported to the plasma membrane.The structural determinants that direct the addition of PIglycan to proteins are only partially understood. Studies of decay-accelerating factor (DAF),I Qa-2, and placental alkaline phosphatase (PLAP) suggest that the signals for PIglycan modification reside in or near the COOH-terminal 1. Abbreviations used in this paper: CRD, cross-reacting determinant ; DAF, decay-accelerating factor ; PLAP, placental alkaline phosphatase ; VSG, variable surface glycoprotein .