The protein encoded by the proto-oncogene c-fos is constitutively nuclear in most cell types analyzed. It has a predicted molecular weight of about 55kDa. Proteins with a molecular weight above 40kDa cannot enter the nucleus passively. Our interest was to study which regions in the protein are involved in the nuclear transport. We prepared a series of deletions and point mutations of the protein and cloned the mutated genes into a eukaryotic expression vector. Cos-1 cells were used to express the mutants transiently. Using indirect immunofluorescence we studied the subcellular localization, analyzing the percentage of cells containing the protein in the nucleus, the cytoplasm, or both locations. Our studies showed that the Fos protein contains several regions which can act independently to translocate the protein into the nucleus.