CYCLE Is a Second bHLH-PAS Clock Protein Essential for Circadian Rhythmicity and Transcription of Drosophila period and timeless.

In 1998 Rosbach, Hall and colleagues discovered the cycle gene, clock gene, and cryptochrome photoreceptor in Drosophila through the use of forward genetics, by first identifying the phenotype of a mutant and then determining the genetics behind the mutation. 
Order of authorship in the original publication: Rutila, Suri, Le, So, Rosbash, Hall. Digital text from cell.com at this link.

See also: Stanewsky, R.; Kaneko, M.; Emery, P.; Beretta, B.; Wager-Smith, K.; Kay, S.A.; Rosash, M.; Hall, J. C.,  "The cry^b Mutation Identifies Cryptochrome as a Circadian Photoreceptor in Drosophila," Cell, 95, 1998, 681-682.