Null mutations are lethal, arresting development with phenotypes ascribed to the lack of proper apoptosis, such that too many cells are present, yielding exencephaly and webbed paws. Fas-induced apoptosis, however, appears unaffected in thymocytes, suggesting Apaf1 works by a different mechanism (J:49841). Apaf1 is essential for the activation of other caspase genes. Some phenotypes in Apaf1 mutants overlap with those in mice carrying mutations in Casp3 and Casp9 (J:49840, J:49841).