Caspase 9 (Casp9) is essential for apoptosis in various cell types, but its role differs depending on the apoptotic stimulus. Mutant mice lacking Casp9 exhibit embryonic lethality and defective brain development due to reduced apoptosis. Casp9-deficient embryonic stem cells (ES) and fibroblasts are resistant to UV and γ-irradiation, while Casp9-deficient thymocytes are resistant to dexamethasone and γ-irradiation but sensitive to UV and anti-CD95-induced apoptosis. Resistance to apoptosis in Casp9-deficient cells is accompanied by retained mitochondrial membrane potential and cytochrome c translocation to the cytosol, suggesting Casp9 acts downstream of cytochrome c. Caspase processing is inhibited in Casp9-deficient ES cells but not in thymocytes or splenocytes. Comparison of Casp9 and Casp3 requirements in different apoptotic settings indicates at least four distinct apoptotic pathways in mammalian cells. Casp9 deficiency leads to defects in cortex and forebrain structures, with increased BrdU-positive cells and expanded ventricular zones. Casp9 is required for PCD during brain development, similar to Casp3. Casp9-deficient ES cells are resistant to multiple apoptotic stimuli, including UV, γ-irradiation, and chemotherapeutic drugs, but not to all stimuli. Casp9 is not required for TNF-induced apoptosis or CTL-mediated cell death in MEFs, but is essential for other apoptotic pathways. Casp9-deficient thymocytes are resistant to dexamethasone and γ-irradiation-induced apoptosis, but not to α-CD95 or UV-induced apoptosis. Casp9 is required for γ-irradiation-induced splenocyte apoptosis and chromatin condensation. Casp9-deficient splenocytes show normal PS changes but aberrant chromatin condensation upon UV or γ-irradiation. Casp9 is required for mitochondrial cytochrome c release, which is essential for PCD. Casp9 deficiency blocks caspase processing and PARP cleavage in ES cells. Casp9 is required for PCD in response to dexamethasone and γ-irradiation in thymocytes, but not for other stimuli. Casp9 and Casp3 are involved in different apoptotic pathways, with Casp9 acting downstream of cytochrome c release. Casp9 deficiency results in resistance to apoptosis induced by γ-irradiation or chemotherapeutic drugs, similar to p53 deficiency. Casp9 and Casp3 are involved in different apoptotic pathways, with Casp9 acting downstream of cytochrome c release. The existence of multiple apoptotic pathways in mammalian cells suggests that Casp9 and Casp3 are involved in different pathways. The study highlights the importanceCaspase 9 (Casp9) is essential for apoptosis in various cell types, but its role differs depending on the apoptotic stimulus. Mutant mice lacking Casp9 exhibit embryonic lethality and defective brain development due to reduced apoptosis. Casp9-deficient embryonic stem cells (ES) and fibroblasts are resistant to UV and γ-irradiation, while Casp9-deficient thymocytes are resistant to dexamethasone and γ-irradiation but sensitive to UV and anti-CD95-induced apoptosis. Resistance to apoptosis in Casp9-deficient cells is accompanied by retained mitochondrial membrane potential and cytochrome c translocation to the cytosol, suggesting Casp9 acts downstream of cytochrome c. Caspase processing is inhibited in Casp9-deficient ES cells but not in thymocytes or splenocytes. Comparison of Casp9 and Casp3 requirements in different apoptotic settings indicates at least four distinct apoptotic pathways in mammalian cells. Casp9 deficiency leads to defects in cortex and forebrain structures, with increased BrdU-positive cells and expanded ventricular zones. Casp9 is required for PCD during brain development, similar to Casp3. Casp9-deficient ES cells are resistant to multiple apoptotic stimuli, including UV, γ-irradiation, and chemotherapeutic drugs, but not to all stimuli. Casp9 is not required for TNF-induced apoptosis or CTL-mediated cell death in MEFs, but is essential for other apoptotic pathways. Casp9-deficient thymocytes are resistant to dexamethasone and γ-irradiation-induced apoptosis, but not to α-CD95 or UV-induced apoptosis. Casp9 is required for γ-irradiation-induced splenocyte apoptosis and chromatin condensation. Casp9-deficient splenocytes show normal PS changes but aberrant chromatin condensation upon UV or γ-irradiation. Casp9 is required for mitochondrial cytochrome c release, which is essential for PCD. Casp9 deficiency blocks caspase processing and PARP cleavage in ES cells. Casp9 is required for PCD in response to dexamethasone and γ-irradiation in thymocytes, but not for other stimuli. Casp9 and Casp3 are involved in different apoptotic pathways, with Casp9 acting downstream of cytochrome c release. Casp9 deficiency results in resistance to apoptosis induced by γ-irradiation or chemotherapeutic drugs, similar to p53 deficiency. Casp9 and Casp3 are involved in different apoptotic pathways, with Casp9 acting downstream of cytochrome c release. The existence of multiple apoptotic pathways in mammalian cells suggests that Casp9 and Casp3 are involved in different pathways. The study highlights the importance