A new study from the laboratory of Hiroki Ueda at the RIKEN Quantitative Biology Center investigates circadian timekeeping with a novel approach to creating genetic knock-out rescue mice. Published in Molecular Cell, the study shows how this technique was used to quickly create numerous mouse lines, each with different mutations in a circadian regulator called CRY1. Studying each mutation and the effects on behavior showed that specific changes to the protein affected the duration of the circadian period.