Dr. Marc K. Hellerstein, associate professor at the University of California, Berkeley, has a method for precisely measuring cell creation. He replaces a hydrogen atom on glucose molecules with the heavier isotope, deuterium and then gives them to patients. Since glucose is a DNA building block, new immune cells will contain deuterium. The method, which already shows that making more cells seems to be the key to fighting the HIV virus, is being commercialized by SpectruMedix Corp. in State College, Pa.