Differences in the protein repertoires between two organisms should reflect differences in their anatomical and physiological complexities. On p. 6317, Vogel et al. analyse the immunoglobulin superfamily (IgSF) genes, which encode proteins involved in cell-cell recognition and communication, in Caenorhabditis elegans and Drosophila melanogaster. They identify 142 IgSF proteins in the fly and 80 in the worm, and show that three-quarters of the fly's IgSF repertoire and one half of the worm's repertoire have emerged since their evolutionary divergence. The fly genome encodes fewer proteins in total than that of the worm, and the researchers suggest that the expansion of particular protein families, including the immunoglobulin superfamily, has contributed to the evolution of the more complex physiology of the fly.