We inherit genes from our fathers and mothers and, for most of our genes, the copies we receive from either parent are equally active. An important exception to this general rule occurs in a process called genomic imprinting, whereby one gene copy is deliberately silenced. These imprinted genes are important in determining how the fetus grows and how infants adapt their physiology to life outside the womb. But the fact that these genes have one copy that is preselected to being silent poses a risk and makes them particularly vulnerable to mutation events, such as occurs in cancer. Imprinted genes behave in this manner because they are marked in different ways in the male and female germ cells. How these genes are so marked is not fully known, and it is important to find out, because if the marking process goes wrong problems in fertility or developmental abnormalities may arise. By analysing a single imprinted gene in some detail, we have discovered an important part of the mechanism in the germ cell marking event. In this research, we wish to understand this mechanism in more detail and we need to show that it may apply generally to imprinted genes. This work will be done in a model system, but it will provide important new insights for human studies.