DNA replication ensure the duplication of the genome prior cell division. RecQL4 is a protein believed to be involved in initiation of DNA replication and in genome stability. We demonstrated that, in human cells, RecQL4 is dispensable for normal replication as no impairment was observed neither in S-phase progression nor in origin firing. A process of unscheduled DNA synthesis has been observed in cells exposed to replication stress induced by the polymerase α inhibitor, aphidicolin. These cells failed to achieved normally DNA synthesis in S phase, leading to mitotic DNA synthesis (MiDAS) to ensure the completion of the genome duplication and the proper chromosome segregation. We mapped and characterized these MiDAS sites at high resolution. Finally, we showed that RecQL4 was necessary for MiDAS. Therefore, we propose that during evolution RecQL4 acquired a different role: to initiate repair-dependent DNA synthesis in mitosis, rather than origin-dependent DNA replication in S phase.