Maintenance of genomic integrity is essential for cell survival. Specifically, during DNA replication cells use a complex network of mechanisms that prevents genomic instability. Recently, we and others identified Wee1, a serine/threonine and tyrosine kinase, as a new modulator of the genomic stability during S phase. Loss of its activity causes a general DNA damage response activation and a decrease in replication fork speed. These effects are counteracted by the downregulation of the endonuclease complex Mus81-Eme1, showing a new link between this endonuclease and Wee1 during DNA replication. Here we discuss the function of Wee1 in genomic stability and its relationship with the Mus81-Eme1 complex.