['cAMP-dependent protein kinase (cAPK), a ubiquitous protein in eukaryotic cells, is one of the simplest members of the protein kinase family. It was the first protein kinase to be crystallized and continues to serve as a biochemical and structural prototype for this family of enzymes. To further understand the conformational changes that occur in different liganded and unliganded states of cAPK, the catalytic subunit of cAPK was crystallized in the absence of peptide inhibitor.', 'The crystal structure of the catalytic subunit of mouse recombinant cAPK (rC) complexed with adenosine was solved at 2.6 A resolution and refined to a crystallographic R factor of 21.9% with good stereochemical parameters. This is the first structure of the rC subunit that lacks a bound inhibitor or substrate peptide. The structure was solved by molecular replacement and comprises two lobes (large and small) which contain a number of conserved loops.'] Does a binary complex of the catalytic subunit of cAMP-dependent protein kinase and adenosine further define conformational flexibility?