In this week's Science, a team of French and German researchers reported the structure of the nucleocapsid of the measles virus, providing new insights into the assembly of the virus' genomic sheath and potentially uncovering new drug targets. The paper offers a look at the interactions between nucleocapsid proteins and viral RNA, and examines why such proteins bind six nucleotides for a measles virus nucleocapsid and seven for a respiratory syncytial virus nucleocapsid.