RB1 and retinoblastoma: Indeed, several studies identified UHRF1 as a direct target of E2F1 in various cell systems [10, 22], and genetic disruption of E2f1 or E2f3 in a murine retinoblastoma model was shown to ablate UHRF1 expression in Rb1-knockout retinae that would otherwise exhibit high UHRF1 expression [21].