Cdt1 is required for loading the replicative DNA helicase MCM2/7, a process known as DNA replication licensing. Here we show that 129 mouse strains express a Cdt1 mutated allele with enhanced licensing activity. The mutation, named Δ(6)PEST, involves a six-amino acid deletion within a previously uncharacterized PEST-like domain. Cdt1 Δ(6)PEST and more extensive deletions exhibit increased re-replication and transformation activities that are independent of the Geminin and E3 ligase pathways. This PEST domain negatively regulates cell cycle-dependent chromatin recruitment of Cdt1 in G2/M phases of the cell cycle. Mass spectrometry analysis indicates that Cdt1 is phosphorylated at sites within the deleted PEST domain during mitosis. This study reveals a conserved new regulatory Cdt1 domain crucial for proper DNA licensing activity and suggests a mechanism by which the presence of Cdt1 in G2/M phases does not lead to premature origin licensing. These results also question the usage of 129 mouse strains for knockout analyses.