Calcium/calmodulin-dependent serine protein kinase (CASK) is generally known as a scaffold protein. Here we show that overexpression of CASK resulted in a reduced rate of cell growth, while inhibition of expression of endogenous CASK via RNA-mediated interference resulted in an increased rate of cell growth in ECV304 cells. To explore the molecular mechanism, we identified a novel CASK-interacting protein, inhibitor of differentiation 1 (Id1) with a yeast two-hybrid screening. Furthermore, endogenous CASK and Id1 proteins were co-precipitated from the lysates of ECV304 cells by immunoprecipitation. Mammalian two-hybrid protein-protein interaction assays indicated that CASK possessed a different binding activity for Id1 and its alternative splicing variant. It is known that Id proteins play important roles in regulation of cell proliferation and differentiation. Thus, we speculate that the regulation of cell growth mediated by CASK may be involved in Id1. Our findings indicate a novel function of CASK, the mechanism that remains to be further investigated. (C) 2005 Elsevier Inc. All rights reserved.