The image on the left is the procapsid (outer protein coat) of herpes simplex virus type 1 (HSV1), the virus that causes cold sores. The image on the right is the mature capsid of the same virus. The major capsid proteins form hexons (lighter blue) and pentons (darker blue) connected by triplexes (green). As the virus matures, the holes in the procapsid ferment to stabilize the mature capsid. Understanding this process can help researchers determine how the virus infects cells and may lead to new treatments.