Almost half of our DNA sequences are made up of jumping genes—also known as transposons. They jump around the genome in developing sperm and egg cells and are important to evolution. But their mobilization can also cause new mutations that lead to diseases, such as hemophilia and cancer. Remarkably little is known about when and where their movements occur in developing reproductive cells, the key process that ensures their propagation in future generations, but can lead to genetic disorders for the hosts.