An important indication that the KRAB/KAP1 system protects genome integrity during the early embryonic period was provided by the demonstration that KAP1 and the KRAB-ZFP ZFP809 are responsible for silencing murine leukemia virus and some other exogenous retroviruses in mouse ES and embryonic carcinoma (EC) cells [31]–[36]. The gene discussed is TRIM28; the disease is embryonal carcinoma.