passages.id: split_6_test_1270_passage
passages.type: progene_text
passages.text: To explore this question , we used embryonic stem ( ES ) cells in which one ( -/+ ) or both ( -/- ) copies of the gene encoding apoptotic protease activating factor 1 ( Apaf-1 ) , a CED-4 homologue , were disrupted by homologous recombination .