Why do females sometimes reject transplants from males? This mysal tery was partially solved recently, when a cell marker that makes male tissues unacceptable to females was identified by two groups of sciento tists separately -one at the 1e Hammersmith Hospital in London and the other at the Leiden 101 University Hospital in the Nether- lands. Researchers found that a short protein chain encoded by a gene called SMCY, present in the Y-chromosome of males and displayed on the cell surface, draws attack by of the immune system of females. The Ire discovery means that scientists can on now devise ways .to prevent transted plant rejection by female receptors (Science, Vo1269, No 5230).