sentence: However , accumulation of ubPCNA does not correlate with Usp1 proteolysis when the progression of replication forks is stalled with HU [ 20 ] , suggesting either a complex regulation of Usp1 activity ( or its subcellular localization ) when cells are exposed to other DNA damaging agents or the existence of at least one another PCNA deubiquitylating enzyme in mammals acting in response to other DNA damaging agents .