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Paracoccidioides brasiliensis is a thermo-dimorphic fungus that causes a human systemic mycosis with high incidence in Latin America. Owing to their participation in the control of pathogen morphogenesis, differentiation and virulence, it was decided to characterize ras genes in P. brasiliensis. ras1 and ras2 were identified to be coding for two different proteins with high identity. The ras transcriptional pattern was investigated by reverse transcription PCR (RT-PCR) during mycelium-to-yeast (M→Y) transition, heat shock at 42 °C and after internalization of yeast cells by murine macrophages. Both genes were downregulated inside macrophages and ras1, at 42 °C. In contrast, ras genes did not show any transcriptional variation during the M→Y transition. The fact that Ras proteins are attached to the membrane via farnesylation prompted the use of a farnesyltransferase inhibitor to investigate the …