In recent years the adult heart has gone from being thought of as a terminally differentiated organ to one with a limited regenerative capacity. The discovery of several putative cardiac stem/progenitor populations including Is11, c-Kit and side population (SP) cells, has led to the possibility of using cellular therapies to treat cardiac injury. We have previously identified SP cells from the adult mouse and human heart that are able to differentiate along a cardiac lineage. SP cells are characterised by their ability to efflux Hoechst 33342 dye due to the expression of Abcg2 (a member of the ABC transporter family). We have hypothesised that the developing fetal heart would be a stem cells rich environment and that SP cells would be present in the human fetal heart and can be isolated from it.