MDM4 and neoplasm: Such a scenario of non-synchronized ‘genomic breathing' in populations of tumor cells might easily contribute to tumor heterogeneity, and is concordant with the model of parallel evolution of different subclones within the same tumor.40, 41 Interestingly, overexpression of MdmX was recently shown to impair DNA damage response and promote chromosome and chromatid breaks in mouse embryo fibroblasts in vitro.