Recently, we demonstrated that the DAN gene product contains a growth- and/or a tumor-suppressive activity in vitro. In the present work, using a yeast two-hybrid system, we searched for cellular proteins that can associate with the DAN gene product. A cDNA clone, termed DA41, was initially isolated from an adult rat lung cDNA library. The DA41 gene was expressed in all adult tissues examined, however, the levels of expression varied significantly among the different tissues. Like the DAN gene product, the DA41 protein was similarly restricted to the cytoplasm. Sequence analysis revealed that DA41 cDNA is 2,167 nucleotides in length and contains a single open reading frame (ORF) of 582 amino acids (61,945 daltons). A homology search revealed that the DA41 gene product shares no structural similarity with those filed in the data base. In a synchronous 3Y1 cell culture, DA41 mRNA was expressed at a low level in quiescent cells; however, its level was significantly increased between the G1 and S phases of the cell cycle. On the other hand, the expression level of DAN mRNA did not change throughout the cell cycle progression. These results suggest that the DAN-DA41 complex might play a crucial role in the regulation of the cell cycle progression.