DNA interruptions made a late entrance.

Until 1977, geneticists had always thought of a gene as one long stretch of DNA that describes how to build a protein. That year, two research groups showed that the DNA instructions for a single protein may frequently be interrupted by introns, fragments of DNA that serve no apparent purpose.

Almost immediately, a group of researchers, dubbed the introns-early crowd and led by Walter Gilbert of Harvard University, proposed that the DNA segments were of ancient origin and vital to the creation of modern genes. They argued that the genes describing today's complex proteins derived from small pieces of DNA called exons. Each exon, the theory went, describes a primitive protein. As time passed, exons linked up in different combinations to encode more complex proteins. Gilbert argued that introns facilitated this "shuffling" of exons.

The introns-early crowd appeared to have won a major victory 2 years ago when investigators discovered a new intron in a gene for an enzyme called TPI. Before 1993, researchers had found only 11 different introns in TPI genes from various species. To fit his model of how exons created the TPI gene, Gilbert argued that a seemingly uninterrupted portion of the gene had once been broken into two exons by an intron. He even predicted where that intron should be--and that's exactly where researchers found one in 1993.

Now, two new studies indicate that the celebration of the intron-early supporters may be short-lived. There are at least seven other novel introns in various TPI genes, reports a group led by Jeffrey D. Palmer of Indiana University in Bloomington and Virginia K. Walker of Queen's University in Kingston, Ontario, in the Aug. 29 Proceedings of the National Academy of Sciences (PNAS). Gilbert's exon-shuffling model for the TPI gene can't account for these new introns, argues Palmer, who says the data favor the idea that introns have been inserted randomly, and recently, into genes.

A second study in the same issue of PNAS, by a group headed by Francisco J. Ayala of University of California, Irvine, launches a similar attack on the introns-early theory, noting that the TPI intron found in 1993 appears in two closely related species of insects but not in 19 other, diverse species. They argue that the insertion of that intron occurred relatively recently in the common ancestor of just a few species of insects.

Gilbert, who says his group will soon present new evidence, retorts that "the last word hasn't been said yet."