Scientists at St. Jude Children's Research Hospital have characterized a hidden intermediate state at the center of Src kinase function. This hidden state allows the kinase to repeatedly modify its target, without needing to release and reattach to the target each time. The researchers have shown that this state is vital to T-cell activation and cell migration, emphasizing the importance of short-lived protein states to major biological processes and opening new avenues for targeting kinases therapeutically.