This domain has the cyclin fold [1] as predicted [2].

1: Evidence for a protein domain superfamily shared by the cyclins, TFIIB and RB/p107. Gibson TJ, Thompson JD, Blocker A, Kouzarides T; Nucleic Acids Res 1994;22:946-952. PMID:8152925

2: Structure of the retinoblastoma tumour-suppressor pocket domain bound to a peptide from HPV E7. Lee JO, Russo AA, Pavletich NP; Nature 1998;391:859-865. PMID:9495340