To determinate whether there was a differential role of RING domain and its E3 ligase in regulation of cancer growth, we compared the capabilities of anchorage-independent growth of XIAP−/−(vector), XIAP−/−(HA-XIAP), XIAP−/−(HA-XIAPΔRING), and XIAP−/−(XIAPH467A) in 0.33% soft agar. The gene discussed is XIAP; the disease is cancer.