A report in this week's issue of Science describes a set of genes that enable members of the bacterial species Proteus mirabilis to tell the difference between kin and strangers. The bacteria engage in a social behavior (common to teenagers and bacteria) called swarming: they like to get together in groups. If you spread these bacteria on a petri dish, they are able to move together to form a colony. The catch is this: like humans, bacteria of the same species can be divided up into smaller populations of more closely related individuals. In bacteria, these smaller groups are called strains. And normal P. mirabilis individuals can somehow tell whether other bacteria are members of the same strain. Members of the same strain will swarm together into one colony, but colonies from different strains will have distinct boundaries between them. How is it that these bacteria know to form a boundary when they bump up against members of a different strain, but not when they meet members of the same strain? To find these answer, a group of researchers from the University of Washington and the University of Iowa used the classic genetic approach: find mutant bacteria that can't do what the normal bacteria do. They found some mutant bacteria that could not recognize members of their own species: they treated all other bacteria as foreigners, even members of their parental strain. These mutant bacteria had mutations in a cluster of similar genes that the researchers called (again in the classic genetic way of naming things we know nothing about) "recognition factors". What these proteins actually do in the cell physically we don't know. But like us, with our immune system proteins that help our cells distinguish "us" from "them", bacteria have proteins that act like aircraft transponders, broadcasting their identity.