When good Cdk5 turns bad.

The cyclin-dependent kinase-5 (Cdk5) is critical to normal mammalian development and has been implicated in synaptic plasticity, learning, and memory in the adult brain. But Cdk-5 activity has also been linked to neurodegenerative diseases. Could a single protein have opposing effects? A new study shows that production of a neuronal protein capable of regulating Cdk-5 activity can turn Cdk-5 from "good" to "bad." The findings may have implications for the development and treatment of conditions like Alzheimer's disease.