4DJ2:
reference: Unwinding the differences of the mammalian PERIOD clock proteins from crystal structure to cellular function., Kucera N, Schmalen I, Hennig S, Ollinger R, Strauss HM, Grudziecki A, Wieczorek C, Kramer A, Wolf E, Proc Natl Acad Sci U S A. 2012 Feb 13. PMID: 22331899

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