In the yeast Saccharomyces cerevisiae translation of apocytochrome b, the only mitochondrially encoded subunit of ubiquinol-cytochrome c oxidoreductase, requires the products of at least three nuclear genes, CBP6, CBS1 and CBS2. In this article I review available data on CBS1p and CBS2p from the initial detection of the genes up to the current investigations on interacting components and the proteins' topology.