Abstract: Methods are provided for identifying whether a tumor, and especially a lung tumor, will be responsive to treatment with a therapeutic regimen that contains a platinum-based agent such as cisplatin and optionally contains a taxane. A specific Schlafen family member 11 (SLFN11) fragment peptide is precisely detected and quantitated by spectrometry directly in lung cancer cells collected from lung tumor tissue obtained from a cancer patient. Comparison to reference levels determines if the cancer patient will respond positively or negatively to treatment with the chemotherapeutic agents taxane plus a platinum-based agent such as cisplatin.