The pRb pathway plays a key role in controlling the G1/S transition in cell cycle progression. Aberrations of various components of the pRb pathway, such as retinoblastoma protein and its upstream actors including cyclin D1, cyclin dependence kinase-4 and p16/p15 cyclin dependent kinase inhibitors, have been reported in a variety of human tumors. Furthermore, the alterations of retinoblastoma protein and its upstream components often occur in a reciprocal manner. Previously, we have reported frequent inactivation of the Cdkn2a/Cdkn2b loci encoding p16/p15 cyclin dependent kinase inhibitors in a subset of 2',3'-dideoxycytidine- and 1, 3-butadiene-induced mouse lymphomas (S.-M. Zhuang, A. Schippert, A. Haugen-Strano, R.W. Wiseman, P. Söderkvist, Inactivation of p16(INK4a)-alpha, p16(INK4a)-beta and p15(INK4b) genes in 2', 3'-dideoxycytidine- and 1,3-butadiene-induced lymphomas, Oncogene 16 (1998) 803-808), indicating the involvement of pRb pathway in lymphomagenesis. To investigate whether alteration of other components in pRb pathway is an alternative mechanism underlying the development of these chemically induced lymphomas, we have examined the genetic status of Rb1, Ccnd1 and Cdk4 genes that encode retinoblastoma protein, cyclin D1 and cyclin dependence kinase-4, respectively. Gross alterations of the Rb1, Ccnd1, and Cdk4 genes were not detected by Southern analysis in any of the tumors examined. In addition, single-strand conformation analysis failed to reveal point mutations in the Cdk4 amino terminal domain that is important for its association with Cdkn2a gene products. These results indicate that the mechanisms underlying the development of 2', 3'-dideoxycytidine- and 1,3-butadiene-induced lymphomas involve inactivation of p16/p15 cyclin-dependent kinase inhibitors but not genomic alterations of the Rb1, Ccnd1 and Cdk4 genes.