Although our fingers and toes are fused when they first form in the womb, we emerge without webbing because the cells that join our digits die. What prompts the cells’ demise? Developmental biologist Lee Niswander and grad student Hongyan Zou, both of New York’s Memorial Sloan-Kettering Cancer Center and Cornell University Medical College, have found out: it’s bone morphogenetic protein (bmp), a growth factor involved in bone and cartilage formation. They injected a mutant gene for a cell-surface receptor for bmp into the right legs of two-day-old chicken embryos. Though chick feet are well supplied with bmp, the faulty receptor could no longer transmit the signal that initiates cell death. The chicks developed webbed toes on their right feet, as shown here. A slight effect is also seen in the left foot because the mutant gene spread there too. It kind of looked like we’d turned a chicken into a duck, says Niswander. (Ducks, they found, don’t produce bmp between their digits.) Niswander hopes her findings will provide insight into tumor formation, where cells don’t die on cue. We know that mutations in this growth-factor family can lead to cancer, she says. If we can get this receptor in there and trigger those cells to die, that would be one application.