Chemists at the pharmaceutical company GlaxoSmithKline had some problems with the route originally devised to install a chiral secondary amine in the drug candidate GSK2879552. To make a key intermediate of the molecule, which is in Phase II clinical trials for the treatment of small-cell lung cancer and acute leukemia, they first had to separate enantiomers of the secondary amine. Then, following reductive amination, they had to use a complex extraction process that required several environmentally unfriendly solvents and generated boron waste. Looking to make their synthesis greener, simpler, and less expensive, the chemists turned to imine reductase enzymes. Such enzymes are known to perform reductive amination reactions, but until now they have been used only with simple amines and carbonyl compounds and in conditions that aren’t amenable to industrial syntheses. The GSK team, led by Gheorghe-Doru Roiban, put an imine reductase enzyme from the bacterium Saccharothrix espanaensis through three rounds of directed evolution, eventually creating an enzyme that’s 38,000 times as good as the original enzyme at catalyzing the desired reductive amination (Nat. Catal. 2019, DOI: 10.1038/s41929-019-0341-4). The enzyme can make kilogram quantities of the intermediate, reduces the number of solvents used in the synthesis from seven to three, and eliminates boron-containing waste from the original route.