cdk3 has been considered to be rate-limiting for cell cycle progression of mammalian cells while its precise function remains to be elucidated. To assess cdk3 function, a cDNA coding for a cyclin-like protein (designated as ik3-1 from an interactor-1 with cdk3) was isolated with the yeast two-hybrid system using a cyclin-dependent kinase 3 (cdk3) cDNA as bait. p70(ik3-1) (a 70-kDa protein designated as p70(ik3-1)) seems to belong to the cyclin family as its C-terminal domain composed of 124 amino acids resembles the highly conserved cyclin box. Coimmunoprecipitation indicated that p70(ik3-1) binds to p35(cdk3) in vivo. The ik3-1 gene may belong to a multigene family and is highly conserved during evolution. mRNA expression of ik3-1 was low in the early G1 phase, upregulated during G1 progression, maximal at a mid-late G1 point, and declined gradually thereafter, suggesting that it may work mainly in G1 phase.