the same time Roshbash's team showed that PER protein is absent in mutant per. In 1994, Young and his team discovered the gene timeless (tim) that influences the activity of per. In 1998, they discovered doubletime (dbt), which regulate the amount of PER protein. In 1990, Konopka, Rosbash, and identified a new gene called Clock (Clk), which is vital for the circadian period. In 1998, they found a new gene cycle (cyc), which act together with Clk. In the late 1998, Hall and Roshbash's team discovered cryb, a gene for sensitivity to blue light. They simultaneously identified the protein CRY as the main light-sensitive (photoreceptor) system. The activity of cry is under circadian regulation, and influenced by other genes such as per, tim, clk, and cyc. The gene product CRY is a major photoreceptor protein belonging to a class of flavoproteins called cryptochromes. They are also present in bacteria and plants. In 1998, Hall and Jae H. Park isolated a gene encoding a neuropeptide named pigment dispersing factor (PDF), based on one of the roles it plays in crustaceans. In 1999, they discovered that pdf is expressed by lateral neurone ventral clusters (LNv) indicating that PDF protein is the major circadian neurotransmitter and that the LNv neurones are the principal circadian pacemakers. In 2001, Young and his team demonstrated that glycogen synthase kinase-3 (GSK-3) ortholog shaggy (SGG) is an enzyme that regulates TIM maturation and accumulation in the early night, by causing phosphorylation. Hall, Rosbash and Young shared the Nobel Prize in Physiology or Medicine 2017 “for their discoveries of molecular mechanisms controlling the circadian rhythm”. Mechanism In Drosophila there are two distinct groups of circadian clocks, namely the clock neurones and the clock genes. They act concertedly to produce the 24-hour cycle of rest and activity. Light is the source of activation of the clocks. The compound eyes, ocelli, and Hofbauer-Buchner eyelets (HB eyelets) are the direct external photoreceptor organs. But the circadian clock can work in constant darkness. Nonetheless, the photoreceptors are required for measuring the day length and detecting moonlight. The compound eyes are important for differentiating long days from constant light, and for the normal masking effects of light, such as inducing activity by light and inhibition by darkness. There are two distinct activity peaks termed the M (for morning) peak, happening at dawn, and E (for evening) peak, at dusk. They monitor the different day