The steps of GPI synthesis were first identified by isolating large numbers of mutant cell lines that had lost the ability to express GPI anchored proteins on their surfaces. Somatic cell hybrid analyses of these lines allowed the definition of complementation groups corresponding to distinct mutated genes, and cDNAs corresponding to normal forms of these genes were identified on the basis of their abilities to restore normal cell surface protein expression in mutant cells. Co-precipitation experiments with tagged cloned proteins have allowed the identification of additional proteins involved in GPI anchor biosynthesis.