KDM5A and KDM5B histone-demethylases are overexpressed in many cancers and have been involved in drug tolerance. Here, we describe that KDM5A, together with KDM5B, contribute to replication stress (RS) response and tolerance. First, they positively regulate RRM2, the regulatory subunit of Ribonucleotide Reductase. Second, they are required for optimal activation of Chk1, a major player of the intra-S phase checkpoint that protects cells from RS. This role in Chk1 activation is probably direct since KDM5A is enriched at ongoing replication forks and associates with both PCNA and Chk1. Because RRM2 is a major determinant of replication stress tolerance, we developed cells resistant to HU, and show that KDM5A/B proteins are required for both RRM2 overexpression and tolerance to HU, in a manner that is independent of their demethylase activity. Altogether, our results indicate that KDM5A/B are major players of RS management. They also show that drugs targeting the enzymatic activity of KDM5 proteins may not affect all cancer-related consequences of KDM5A/B overexpression.KDM5 proteins belong to the JUMONJI family of histone demethylases that catalyze the demethylation of di-and tri-methylated lysines on histone and non-histone proteins. KDM5A, also known as JARID1A or RBP2 (Retinoblastoma binding protein 2), was originally discovered as a Retinoblastoma binding protein [1]. Further studies showed that KDM5A is a histone-demethylase specific to the di-and tri-methylated forms of Lys4 of Histone H3 (H3K4me2/3), a mark associated with promoters of transcriptionally active genes [2][3][4]. In agreement with its demethylase activity against an active mark of transcription, KDM5A is involved in gene repression by demethylating H3K4me3 at gene promoters. It participates in many repressive chromatin complexes, including the Polycomb complex [5], the Sin3 corepressor complex [6] and the NuRD complex [7]. KDM5A was involved in the stable repression of E2F dependent genes that occurs during terminal differentiation and senescence, two biological processes requiring permanent exit from the cell cycle [6,8]. Rb that is up-regulated during differentiation was shown to sequestrate KDM5A/Rbp2, thus impeding its repressor activity on genes required for differentiation [9]. Whereas its role in the stable repression of E2F dependent genes has been well documented, whether it is also important for the regulation of the oscillation of E2F genes transcription during the cell cycle, is less known. However, a recent report describes its interaction with p130 in order to demethylate H3K4 on E2F promoters in G0 and early G1 [10].KDM5A can also act as a transcriptional activator. When enriched in gene bodies, it plays a role in the elongation step of Polymerase II (Pol-II) by maintaining low levels of H3K4 methylation [11]. In addition, KDM5A activates genes critical for mitochondrial function and metabolism, in a manner that is independent of its demethylase activity but dependent of its C-terminal PHD3 domain that serves as docking site to H...