The Jun oncoprotein is a major component of the transcription factor complex AP-1, which regulates the expression of multiple genes essential for cell proliferation, differentiation and apoptosis. Constitutive activation of endogenous AP-1 is required for tumor formation in avian and mammalian cell transformation systems, and also occurs in distinct human tumor cells suggesting that AP-1 plays an important role in human oncogenesis. The highly oncogenic v-jun allele capable of inducing neoplastic transformation in avian fibroblasts and fibrosarcomas in chicken as a single oncogenic event, was generated by mutation of the cellular c-jun gene during retroviral transduction. Hence, avian cells represent an excellent model system to investigate molecular mechanisms underlying jun-induced cell transformation. Approaches aimed at the identification of genes specifically deregulated in jun-transformed fibroblasts have led to the identification of several genes targeted by oncogenic Jun. Some of the activated genes represent direct transcriptional targets of Jun encoding proteins, which are presumably involved in cell growth and differentiation. Genes suppressed in v-jun-transformed cells include several extracellular proteins like components of the extracellular matrix or proteins involved in extracellular signalling. Due to aberrant regulation of multiple genes by the Jun oncoprotein, it is assumed that only the combined differential expression of Jun target genes or of a subset thereof contributes to the conversion of a normal fibroblast into a tumor cell displaying a phenotype typical of jun-induced cell transformation. It has already been shown that distinct activated targets exhibit partial transforming activity upon over-expression in avian fibroblasts. Also, distinct target genes silenced by v-Jun inhibit tumor formation when re-expressed in v-jun-transformed cells. The protein products of these transformation-relevant genes may thus represent potential drug targets for interference with jun-induced tumorigenesis.