Benzer and student Ron Konopka discovered the first circadian rhythm mutants. Three distinct mutant types—arrhythmic, shortened period, and lengthened period—were identified. These mutations all involved the same functional gene on the X chromosome and influenced the eclosion rhythm of the population as well as rhythms in individual flies' locomotor activity. To monitor Drosophila locomotor activity, Benzer and postdoctoral researcher, Yoshiki Hotta, designed a system using infrared light and solar cells. All three mutations were mapped to the X chromosome, zero centimorgans away from each other, indicating that the mutant phenotypes corresponded to alleles of the same gene, which Konopka named period. This was the first of several seminal studies of single genes affecting behavior, studies that have been replicated in other animal models and are now the basis for the growing field of molecular biology of behavior. In 1992 Benzer, working with Michael Rosbash, furthered this work by showing that the PER protein, which period codes for, is predominantly located in the nucleus. The work with Period mutants was catalytic in the study of circadian rhythms and served to propel the field forward.