Reports on the article by Philip D. Jeffrey et al published in the July 27, 1995 issue of `Nature' magazine describing the crystal structure of a partially active cyclin A-cyclin dependent kinase 2 (CDK2) complex. Similarities between the proteins of the retinoblastoma family and the transcription factor TFIIB; Role of the cyclin-CDK complexes in the initiation of DNA replication and mitosis.