discovered the importance of light-dark conditions in D. pseudoobscura. He demonstrated that eclosion rhythm was delayed but not stopped when temperature was decreased. He concluded that temperature influenced only the peak hour of the rhythm, and was not the principal factor. It was then known that the circadian rhythm was controlled by a biological clock. But the nature of the clock was then a mystery. After almost two decades, the existence of the circadian clock was discovered by Seymour Benzer and his student Ronald J. Konopka at the California Institute of Technology. They discovered that mutations in the X chromosome of D. melanogaster could make abnormal circadian activities. When a specific part of the chromosome was absent (inactivated), there was no circadian rhythm; in one mutation (called perS, "S" for short or shortened) the rhythm was shortened to ~19 hours; whereas, in another mutation (perL, "L" for long or lengthened) the rhythm was extended to ~29 hours, as opposed to a normal 24 hour rhythm. They published the discovery in 1971. They named the gene location (locus) as period (per for short) as it controls the period of the rhythm. In opposition, there were other scientists that stated genes could not control such complex behaviors as circadian activities. Another circadian behavior in Drosophila is courtship between the male and female during mating. Courtship involves a song accompanied by a ritual locomotory dance in males. The main flight activity generally takes place in the morning and another peak occurs before sunset. Courtship song is produced by the male's wing vibration and consists of pulses of tone produced at intervals of approximately 34 msec in D. melanogaster (48 msec in D. simulans). In 1980, Jeffrey C. Hall and his student Charalambos P. Kyriacou, at Brandeis University in Waltham, discovered that courtship activity is also controlled by per gene. In 1984, Konopka, Hall, Michael Roshbash and their team reported in two papers that per locus is the centre of the circadian rhythm, and that loss of per stops circadian activity. At the same time, Michael W. Young's team at the Rockefeller University reported similar effects of per, and that the gene covers 7.1-kilobase (kb) interval on the X chromosome and encodes a 4.5-kb poly(A)+ RNA. In 1986, they sequenced the entire DNA fragment and found the gene encodes the 4.5-kb RNA, which produces a protein, a proteoglycan, composed of 1,127 amino acids. At