Using a NanoBRET quantitative ubiquitination assay to examine the binding of ubiquitin to XIAP upon treatment with A4, neuroblastoma cells were observed to exhibit a clear dose-dependent increase in BRET ratio, indicating an increase binding of fluorescent-labeled ubiquitin to the endogenous luminescent-tagged XIAP (Fig. 3C). This evidence concerns the gene XIAP and neuroblastoma.