Maintaining genomic stability in a continually dividing cell population requires accurate DNA repair, especially in male germ cells. Repair and replication protein access to DNA, however, is complicated by chromatin compaction. The HP1β chromatin protein, encoded by Cbx1, is associated with chromatin condensation but its role in meiosis is not clear. To investigate the role of Cbx1 in male germ cells, we generated testis specific Cbx1 deficient transgenic mice by crossing Cbx1 flox/flox (Cbx1 f/f ) mice with Stra8 Cre +/mice. Loss of Cbx1 in testes adversely affected sperm maturation and Cbx1 deletion increased seminiferous tubule degeneration and basal level DNA damage., We observed that Cbx1 -/-MEF cells displayed reduced resolution of stalled DNA replication forks as well as decreased fork restart, indicating defective DNA synthesis. Taken together, these results suggest that loss of Cbx1 in growing cells leads to DNA replication defects and associated DNA damage that impact cell survival.