The human nucleolus organizer regions

Although the nucleolus was first described in the early 19th century from both animal and plant cells, human nucleoli and particularly the five human nucleolus organizers have not been well characterized. In this issue of Genes & Development, van Sluis and colleagues (pp. 1688-1701) present a detailed molecular analysis of these organizers, which occur on the short arms of five human chromosomes. The near identity of these arms suggests extensive interchromosomal exchange during evolutionary history.