The origins of introns and their evolutionary role remain unclear. In the November 6 Proceedings of the National Academy of Science, Fedorov et al. describe the use of a computer program, called INTRONMAP to present evidence for the existence of ancient introns (Proc Natl Acad Sci USA 2001, 98:13177-13182). They used the program to map intron positions onto homologous genes encoding proteins for which the three-dimensional structure is known. They applied the program to 665 nonredundant protein sequences in the Protein Data Bank and mapped over 8,000 introns. The result was a correlation of phase-zero introns with module boundaries in proteins, corresponding to ancient conserved regions (ACR). There was no correlation for phase-one or phase-two positions, or for non-ACR proteins (which presumably represent 'modern' genes).