NALM-6 is a pre-B leukemia cell line sensitive to exogenous nitric oxide (NO), which enters into apoptosis during 24 h of exposure to low doses of the NO donors SNAP (100 microM) or DETA-NO (250 microM). By culturing NALM-6 with repeated and increasing concentrations of SNAP, we obtained a variant (NALM-6R) that retains >95% viability and does not enter into apoptosis during 24 h culture in the presence of up to 500 microM SNAP or 750 microM DETA-NO. A power blot screen performed with 277 antibodies on cell lysates from NALM-6 and NALM-6R cultured without NO donors served to determine the altered constitutive expression of 19 proteins in NALM-6R. Proteins affected in the less sensitive cell line NALM6-R are involved in the regulation of apoptosis, the cell cycle, cell interactions, signal transduction, cell morphology, and cell motility. This model shows that repeated exposure of tumor cells to NO may either select NO-resistant cells or contribute to NO-sensitive conversion into NO-resistant cells. The identification of the proteins that are affected during this transition may help us to define the mechanisms that are involved in cell resistance to NO-cytotoxicity which often accompany clinical progression.