The function of ENSG00000063438 (AHRR, aryl hydrocarbon receptor repressor) is as follows. Mediates dioxin toxicity and is involved in regulation of cell growth and differentiation. Represses the transcription activity of AHR by competing with AHR for heterodimer formation with ARNT and subsequently binding to the xenobiotic response element (XRE) sequence present in the promoter regulatory region of a variety of genes. Represses CYP1A1 by binding the XRE sequence and recruiting ANKRA2, HDAC4 and/or HDAC5. Autoregulates its expression by associating with its own XRE site. {ECO:0000269|PubMed:17890447, ECO:0000269|PubMed:18172554}. [Isoform 1]: Represses AHR transcription activity (PubMed:19380484). Also represses transcription mediated by HIF1A and EPAS1/HIF2A but does not repress NR1I2/PXR or ESR1 transcriptional activities (PubMed:19380484). . [Isoform 2]: Does not repress AHR transcription activity. .