The acyl carrier protein (ACP), aryl carrier protein (ArCP), and the peptidyl carrier protein (PCP) are a family of protein cofactors that participate in fatty acid (acyl), polyketide, and nonribosomal peptide (peptidyl) biosynthesis. The growing molecule is bound to the A(r)/PCP via a thioester derived from the distal thiol of a 4'-phosphopantetheine (PPant) moiety.