CHK1: gene encodes for a serine/threonine kinase involved in the regulation of cell cycle progression and DNA damage checkpoints. To determine the role of CHK1 in the pathogenesis of lymphoid neoplasms and its relationship to other DNA damage response genes, we have analyzed the gene status, protein, and mRNA expression in a series of tumors and nonneoplastic lymphoid tissues. CHK1 protein and mRNA expression levels were very low in both reactive tissues and resting lymphoid cells, whereas tumor samples showed a variable pattern of expression related to their proliferative activity. However, seven aggressive tumors showed a dissociate pattern of extremely low or negative protein expression in spite of a high proliferative activity. Four of these tumors were diffuse large B-cell lymphomas (DLCLs) with concordant reduced levels of mRNA, whereas one blastoid mantle cell lymphoma (B-MCL) and two DLCLs had relatively normal levels of mRNA. No gene mutations, deletions, or hypermethylation of the promoter region were detected in any of these cases. In all these tumors ATM, CHK2, and p53 genes were wild type. These findings suggest that CHK1 inactivation in NHLs occurs by loss of protein expression in a subset of aggressive variants alternatively to ATM, CHK2, and p53 alterations.