Recognizes a 27kDa protein, identified as the p27Kip1, a cell cycle regulatory mitotic inhibitor. Its epitope spans between aa 83-204 of p27. It is highly specific and shows no cross-reaction with other related mitotic inhibitors. p27Kip1 functions as a negative regulator of G1 progression and has been proposed to function as a possible mediator of TGF- induced G1 arrest. p27Kip1 is a candidate tumor suppressor gene. This MAb co-precipitates cdk4 in complex p27Kip1 and is excellent for staining of formalin-fixed tissues.