Ubiquitin cooperation with other post-translational modifications (PTMs) provides a tiered opportunity for protein regulation. Though we often consider PTM crosstalk as distinct modification sites, or perhaps at the level of modified ubiquitin (e.g., phospho-ubiquitin), we have recently described the direct ubiquitylation of another PTM, ADP-ribosylation. We refer to this hybrid PTM as mono-ADP-ribosyl ubiquitylation, or MARUbylation. The discovery of cellular MARUbylation establishes an exciting new link between PARP and ubiquitin biology. We will describe our most recent progress in understanding the specific regulation and function of MARUbylation as a novel hybrid PTM.