Organic compounds containing silicon are important for a number of applications, from polymers to semiconductors. The catalysts used for creating carbon-silicon bonds, however, often require expensive trace metals or have limited lifetimes. Borrowing from the ability of some metallo-enzymes to catalyze other rare carbene insertion reactions, Kan et al. used heme proteins to form carbon-silicon bonds across a range of conditions and substrates (see the Perspective by Klare and Oestreich). Directed evolution experiments using cytochrome c from Rhodothermus marinus improved the reaction to be 15 times more efficient than industrial catalysts. Science , this issue p. [1048][1]; see also p. [970][2] Enzymes that catalyze carbon–silicon bond formation are unknown in nature, despite the natural abundance of both elements. Such enzymes would expand the catalytic repertoire of biology, enabling living systems to access chemical space previously only open to synthetic chemistry. We have discovered that heme proteins catalyze the formation of organosilicon compounds under physiological conditions via carbene insertion into silicon–hydrogen bonds. The reaction proceeds both in vitro and in vivo, accommodating a broad range of substrates with high chemo- and enantioselectivity. Using directed evolution, we enhanced the catalytic function of cytochrome c from Rhodothermus marinus to achieve more than 15-fold higher turnover than state-of-the-art synthetic catalysts. This carbon–silicon bond-forming biocatalyst offers an environmentally friendly and highly efficient route to producing enantiopure organosilicon molecules. [1]: /lookup/doi/10.1126/science.aah6219 [2]: /lookup/volpage/354/970?iss=6315