Cycle inhibiting factors (Cif) are bacterial effectors that interfere with the eukarytoc cell cycle. CIF induce an irreversible cell cycle arrest upon injection into host cell. CIF blocks degradation of cyclin -dependent kinase inhibitors p21 and p27, inducing their accumulation in the cell. The x-ray crystal structure of Cif reveals it to be a divergent member of a superfamily of enzymes including cysteine proteases and acetyltransferases.

1: Cycle Inhibiting Factors (Cifs). Cyclomodulins That Usurp the Ubiquitin-Dependent Degradation Pathway of Host Cells Taieb F, Nougayrede JP, Oswald E; Toxins 2011;3:356-368; PMID:22069713