Protein that belongs to the cyclin family or that contains a cyclin box-like domain. Cyclins are regulatory subunits of the cyclin-dependent protein kinases. They form kinase holoenzymes, with distinct biochemical characteristics and nonredundant biological functions, which mediate phosphorylation of cellular proteins, including key cell cycle regulatory molecules. In this way, the kinase holoenzymes promote the transit of cells through the division cycle. Cyclins accumulate during interphase of eukaryotic cell cycle and are destroyed at the end of mitosis.