The second approach takes advantage of mutations in genes required for the efficient engulfment of apoptotic cells (see below). In these mutants, cells still die, but many dying cells fail to be engulfed and removed from the animal. These per sistent, undegraded cell corpses are very obvious, even to the worm neophyte, and thus can be used as a simple assay for the extent of programmed cell death in the animal (Ellis et a l , 1991; Vaux et a l , 1992). Elimination of programmed cell death results in the absence of persistent cell corpses in these mutants. The main advan tage of this assay is its ease of scoring. However, the number of persistent cell corpses is more variable than the number of surviving cells, and weak effects on cell death cannot be detected with this method.