Although cyclin-dependent kinase 5 (Cdk5) is widely expressed in human tissues, its activator p35Nck5a is generally considered to be neuron specific. In addition to neuronal cells, active Cdk5 complexes have been reported in developing tissues, such as the embryonic muscle and ocular lens, and in human leukemia HL60 cells induced to differentiate by an exposure to 1,25-dihydroxyvitamin D(3); however, its activator in these cells has not been demonstrated. The results of this study indicate that p35Nck5a is associated with Cdk5 in monocytic differentiation of hematopoietic cells. Specifically, p35Nck5a is expressed in normal human monocytes and in leukemic cells induced to differentiate toward the monocytic lineage, but not in lymphocytes or cells induced to granulocytic differentiation by retinoic acid. It is present in a complex with Cdk5 that has protein kinase activity, and when ectopically expressed together with Cdk5 in undifferentiated HL60 cells, it induces the expression of CD14 and "nonspecific" esterase, markers of monocytic phenotype. These observations not only indicate a functional relationship between Cdk5 and p35Nck5a, but also support a role for this complex in monocytic differentiation. (Blood. 2001;97:3763-3767)