H2BC21 and Nephroblastoma: At the Polycomb target loci WT1, EN2, HOXD, and HOXB, which encode developmentally regulated transcription factors (Wilms tumor, Engrailed 2, and homeobox transcription factors, respectively), we inserted the 5xtetO-H2B-CITRINE reporter 3 to 4.5 kb upstream of endogenous promoter regions (Fig. 1A and fig.