The Ras converting enzyme (Rce1p) is an isoprenylation specific endoprotease involved in the post-translational processing of isoprenylated proteins, such as Ras and yeast mating pheromone a-factor. Inhibition of the enzyme leads to attenuation of transforming effects of mutant activated Ras and partial mislocalization of the protein, without affecting cell viability. Rce1p is an integral membrane protein localized on the endoplasmic reticulum membrane. There has not yet been any successful derivation of its protein structure using conventional techniques. This study has used the substituted cysteine accessibility method for determining the membrane topology of Rce1p. Two putative topology models have thus been constructed for Rce1p, bearing either five or seven transmembrane spans. Topology determination for Rce1p is a step forward in the determination of its structure and catalytic mechanism, which in turn has implications in the development of Rce1p inhibitors as novel anti-cancer drugs targeting the maturation of Ras protein.