A recent study examined the nucleation of pathologically expanded polyQ tracks and suggested that it involves segments of three glutamine (Q) residues at every other position. Molecular simulations suggested a pattern encoding a four-stranded steric zipper with interdigitated glutamine side chains. Once the initial zipper is formed, it engages naive polypeptides in a fashion characteristic of polymer crystals with intramolecular nuclei. By revealing the physical nature of the rate-limiting event for polyQ aggregation in cells, these findings elucidate the molecular etiology of polyQ diseases.