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In page Period (gene):

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The period gene and three mutants (perS, perL, and per0) were isolated in an EMS mutagenesis screen by Ronald Konopka and Seymour Benzer in 1971.[1] The perS, perL, and per0 mutations were found to not complement each other, so it was concluded that the three phenotypes were due to mutations in the same gene.[1] The discovery of mutants that altered the period of circadian rhythms in eclosion and locomotor activity (perS and perL) indicated the role of the per gene in the clock itself and not an output pathway. The period gene was first sequenced in 1984 by Michael Rosbash and colleagues.[2] In 1998, it was discovered that per produces two transcripts (differing only by the alternative splicing of a single untranslated intron) which both encode the PER protein.[3]