What guides the growth of neurons, the basic cells of the nervous system, in embryonic human beings? Neurobiologists at the University of California recently identified 2 proteins that enable some 10 trillion neurons to grow at appropriate spots (The Sciences, Sept-Oct 1995).

Named netrin and semaphorin, these guidance proteins work like road signals, directing neurons to their destinations. Researchers have also pinpointed the genes that encode these proteins. This makes it significant clinically, as the comparison of genetic sequences across species would enable patients with nervous disorders to benefit from genes of other animals.