3EJ1:
reference: Synthesis and evaluation of pyrazolo[1,5-b]pyridazines as selective cyclin dependent kinase inhibitors., Stevens KL, Reno MJ, Alberti JB, Price DJ, Kane-Carson LS, Knick VB, Shewchuk LM, Hassell AM, Veal JM, Davis ST, Griffin RJ, Peel MR, Bioorg Med Chem Lett. 2008 Sep 24. PMID: 18835709

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