sentences:  Selection for specific tyrosine residues requires a generally accessible tyrosine residue, and acidic residues within +5 or -5 residues of the target tyrosine.P-selectin glycoprotein ligand-1 (PSGL-1) has been extensively studied as a substrate for TPST and the importance of sulfation in PSGL-1 and its ability to bind its receptor.Another substrate for TPST, CC-chemokine Receptor 5 (CCR5), has generated interest because of its role as the target protein for the viral entrance of HIV into cells.