For many years, it was been believed that we were born with all the brain cells we would ever have, and that new ones could not be formed. More recently, it has been shown that the adult human brain creates new neurons, a process known as neurogenesis. The most active area of neurogenesis is the hippocampus, an area of the brain important for learning and memory. Although the exact role of the newly created neurons is uncertain, to play a part in the acquisition and storage of memories they must locate themselves correctly and be able to communicate with pre-existing circuits in the brain. A new study published in PLoS Biology shows that cyclin-dependent kinase 5 (CDK5) plays a key role in this integration process. The scientists used retroviruses to alter the activity of CDK5, and found that more than 50% of CDK5-deficient cells failed to migrate to the right position in the brain or make the correct connections with surrounding neurons. Instead, the CDK5-deficient cells put out processes in the wrong direction and formed connections with the wrong cells. Cells that fail to integrate correctly could interfere with normal information processing. By increasing understanding of the factors needed for newly formed neurons to become properly integrated into brain, the study may suggest ways to increase the success of neural transplantation to treat brain injuries or diseases such as Parkinson’s disease.