The human 57 kDa Ki-1 antigen (Ki-1⁄57) is a cytoplasmic and nuclear protein, associated with Ser⁄Thr protein kinase activity, and phosphorylat-ed at the serine and threonine residues upon cellular activation. We have shown that Ki-1⁄57 interacts with chromo-helicase DNA-binding domain protein 3 and with the adaptor⁄signaling protein receptor of activated kinase 1 in the nucleus.