TP53(tumour protein 53) is one of the most frequently mutated genes in human cancer and its role during cellular transformation has been studied extensively. However, the homeostatic functions of p53 are less well understood. Here, we explore the molecular dependency network of TP53 through an RNAi-mediated synthetic interaction screen employing two HCT116 isogenic cell lines and a genome-scale endoribonuclease-prepared short interfering RNA library. We identify a variety of TP53 synthetic interactions unmasking the complex connections of p53 to cellular physiology and growth control. Molecular dissection of the TP53 synthetic interaction with UNRIP indicates an enhanced dependency of TP53-negative cells on small nucleolar ribonucleoprotein (snoRNP) assembly. This dependency is mediated by the snoRNP chaperone gene NOLC1 (also known as NOPP140 ), which we identify as a physiological p53 target gene. This unanticipated function of TP53 in snoRNP assembly highlights the potential of RNAi-mediated synthetic interaction screens to dissect molecular pathways of tumour suppressor genes.More than 30 years of intense research on the tumour suppressor gene TP53 has revealed its relevance in many aspects of tumour biology (reviewed in ref. 1). It is now well established that p53 (the protein product of TP53) activation leads to the execution of a complex genetic program that limits cellular proliferation and leads to senescence or apoptosis 2,3 . Several lines of evidence have pointed to an under-appreciated homeostasis function of p53 in normal, unstressed cells 4 . As a transcription factor, p53 can mediate the expression levels of its target genes, thereby adjusting cellular homeostasis of basic metabolism 5 , balance of reactive oxygen species 6,7 , stem cell identity and reprogramming 8 and animal reproduction 9 . These data establish an important role for p53 in cellular homeostasis and indicate that p53-loss-of-function cells become genetically rewired in a way that discriminates them from wild-type cells, a premise that could be explored for the identification of possible p53 genetic dependencies.Synthetic interaction screens have led the way in identifying new and unanticipated functional interactions between seemingly unrelated protein complexes in yeast 10,11 . In mammalian cells, RNA interference (RNAi)-based synthetic interaction screens are a promising approach to dissect genetic dependencies and the first studies on mammalian synthetic interactions of oncogenes have demonstrated the power of this approach 12,13 . Furthermore, synthetic lethal screens have identified genes that sensitize cells to different chemical drugs 14,15 . Several synthetic interactions with tumour suppressor genes have also been published 11,16 , but to our knowledge a genome-scale survey for synthetic interactions of a tumour suppressor gene has not been reported. Here, we present the results of a systematic investigation of TP53 genetic dependencies and characterize in molecular detail a homeostatic function of p53 in snoRNP biogen...