Line 10T1/2 mouse fibroblast overexpressing the v-Jun oncoprotein were morphologically altered, grew into multilayered foci in culture and formed colonies when suspended in agar. The growth rate of the v-Jun-transformed 10T1/2 cells was not changed significantly from that of the untransformed parental cells, but the saturation density of the transformed cultures exceeded that of normal controls by a factor of 2. mRNA extracted from v-Jun-transformed 10T1/2 cells was analysed for differential gene expression with DNA micro-array technology. One of the targets downregulated by v-Jun was identified as SSeCKS (Src-suppressed C kinase substrate). Re-expression of SSeCKS in v-Jun-transformed fibroblasts reversed the transformed phenotype of the cells. Their ability to form foci was reduced to background levels, the number and size of agar colonies was lowered by a factor of 10 and the saturation density was significantly diminished. However, expression of SSeCKS had little effect on the morphology of v-Jun-transformed 10T1/2 cells. These data suggest that the SSeCKS protein has growth-attenuating properties. Down-regulation of SSeCKS may be essential for Jun-induced transformation.