Our bodies follow a 24-hour 'biorhythm' that affects most of our biological functions. This fact forms the basis of cancer chronotherapy, which takes time of day into account to plan cancer treatment. Administering cancer drugs at the right time can double effectiveness while reducing toxicity up to fivefold. However, individual differences in biorhythms mean that the 'right time' varies from one person to another. In a recent study, researchers linked gene expression in mice with the time point at which the chemotherapy agent irinotecan (Camptosar) produced the least toxicity. They developed a mathematical model that predicts each animal's ideal time point based on the expression of two genes. In the future, they hope to develop similar tools to help predict the best time for cancer treatment in human patients.