In a female heterozygous for the A and B genes at the X-linked glucose-6-phosphate dehydrogenase (G-6-PD) locus, single cells express the activity of only one or the other gene. Thus, this system can be used to trace the cellular origin of tumors. Neoplasms arising from a single cell should exhibit only Type A or B enzyme, whereas those with multiple cell origin can show both enzyme types. Each of 14 neurofibromas studied from two G-6-PD heterozygotes with hereditary neurofibromatosis exhibited both Type A and Type B enzyme, strongly suggesting that hereditary neurofibromas have a multiple cell origin. A minimal estimate of the starting number of cells for a neurofibroma is 150, but the number may be as high as several thousand. This suggests that the initial tumorigenic event affects a relatively large number of cells simultaneously or that it affects one or a few cells, subsequently altering the pattern of growth in adjacent cells.