Protooncogene- and tumor-suppressor gene proteins serve essential functions in the regulation of proliferation and differentiation of cells. Abnormal regulation or mutation of these genes, or transformation with retroviral homologs, may lead to tumor development in animals. In contrast to vertebrates, only few data on these genes exist in plants and fungi. Plant nuclear protooncogene homologs, such as myb and myc have multiple regulatory functions in metabolic pathways not existing in mammalian cells; they are involved in the complex regulation of anthocyanin (purple pigment) and phlobaphene (red pigment) biosynthesis, lignin production, trie home differentiation, dehydration stress gene expression and seed development. Apart from these well-characterized roles in plant-specific pathways, few experimental data have been reported on a functional significance in growth and development. A screening for nuclear protooncogene- and tumor-suppressor gene-related proteins in the myxomycete Physarum polycephalum revealed the existence of homologs of vertebrate c-myc, c-fos, c-jun, p53, and retinoblastoma proteins during the synchronous cell cycle or sclerotization. The p53 homologs of Physarum and Zea mays were shown to be specific for quiescent stages of their life cycles. Plants and lower eukaryotes, such as fungi, may be useful experimental systems to elucidate novel functions of protooncogene- and tumor-suppressor proteins in cell cycle regulation and development, or to reveal target genes that might be difficult to identify in complex mammalian systems. Recent data indicate that oncogenes and tumor suppressors in animals have more cellular targets than originally proposed; some of these might be as unexpected as in plant secondary metabolism.