DNA tumour viruses have evolved a number of mechanisms by which they deregulate normal cellular growth control. We have recently described the properties of a cyclin encoded by human herpesvirus 8 (also known as Kaposi's sarcoma‐associated herpesvirus) which is able to resist the actions of p16 Ink4a, p21 Cip1 and p27 Kip1 cdk inhibitors. Here we investigate the mechanism involved in the subversion of a G 1 blockade imposed by overexpression of p27 Kip1. We demonstrate that binding of K cyclin to cdk6 expands the substrate repertoire of this cdk to include a number of substrates phosphorylated by cyclin–cdk2 complexes but not cyclin D1–cdk6. Included amongst these substrates is p27 Kip1 which is phosphorylated on Thr187. Expression of K cyclin in mammalian cells leads to p27 Kip1 downregulation, this being consistent with previous studies indicating that phosphorylation of p27 Kip1 on Thr187 triggers its downregulation. K cyclin expression is not able to prevent a G 1 arrest imposed by p27 Kip1 in which Thr187 is mutated to non‐phosphorylatable Ala. These results imply that K cyclin is able to bypass a p27 Kip1‐imposed G 1 arrest by facilitating phosphorylation and downregulation of p27 Kip1 to enable activation of endogenous cyclin–cdk2 complexes. The extension of the substrate repertoire of cdk6 by K cyclin is likely to contribute to the deregulation of cellular growth by this herpesvirus‐encoded cyclin.