While the bipartite region discovered accounted for, 66% of HER3’s transactivation potential in a Gal4 UAS-luciferase assay. To identify how B1 and B2 influence nuclear HER3 function, we first demonstrated that HER3 can bind to a 122 bp region of the cyclin D1 promoter, a region that was originally found to associate with nuclear EGFR. Interestingly, EGFR, HER2, and HER3 all associated with this relatively small promoter region in SKBr3 cells. Whether HER family dimers exist in the nucleus and function together as co-transcription factors has yet to be determined. We further demonstrate that HER3 lacking the B1 and B2 regions had either reduced ability or an inability to transactivate cyclin D1 promoter-luciferase and cyclin D1 expression in multiple cell lines, suggesting that these regions function as prominent TADs. SCC6 and BT474 cells express endogenous HER3 and therefore the slight increases in reporter activity detected upon overexpression of HER3DB1DB2 may have been due to activation by endogenous nuclear HER3. Additionally, both HER3WT and HER3DB1DB2 were effectively localized to the nucleus. Therefore, the lack of cyclin D1 promoter-luciferase detected upon HER3DB1DB2 overexpression was likely due to its inability to function in the nucleus rather than impairment in nuclear translocation. The specific transcription factors that associate with nuclear HER3 is under current investigation, but we speculate that HER3DB1DB2 is deficient in the proper association and/or recruitment of these factors. Collectively, these data suggest that the B1 and B2 regions of HER3 function as TADs. One 
of the major hurdles in the study of nuclear RTKs is to experimentally isolate their nuclear functions from plasma membrane-bound functions. To ensure that the loss of cyclin D1 promoter- luciferase activity detected upon overexpression of HER3DB1DB2 was due to Albaspidin-AA deficiency in nuclear HER3 functions the tyrosine residues located in the B1 and B2 regions known to play a role in activating signaling cascades were mutated. HER3 mutated at both tyrosine 1222 and 1289 was only slightly hindered in the activation of the cyclin D1 promoter, Mepiroxol unlike HER3DB1DB2, and both HER3DM and HER3DB1DB2 were still capable of activating HER3’s downstream effector kinase AKT. To further validate that the regulation of cyclin D1 was not a result of classical membranebound functions of HER3, ICD mutants of HER3 were created in which HER3WT and HER3DB1DB2 were deleted of both the Nterminus and transmembrane domain. The WT-ICD, which cannot be localized at the plasma membrane or serve as a dimerization partner to activate classical signaling pathways, was still capable of regulating cyclin D1 luciferase activity and mRNA expression. This finding falls in line with the identified HER3 Cterminal splice variants that have been shown to function as cotranscription factors. Importantly, DB1DB2-ICD was hindered in cyclin D1 regulation, further supporting the role of these TADs in influencing nuclear HER3 transcriptional function. We speculate that the minor increases in luciferase and mRNA expression observed upon DB1DB2-ICD overexpression may emanate from endogenous HER3 in SCC6 and BT474 cells and/or the residual transcriptional activity remaining on the Cterminus of the DB1DB2-ICD. Collectively, these data suggest that the loss of cyclin D1 promoter-luciferase activity and mRNA expression was not due to modulation of signaling pathways emanating from membrane-bound HER3, but likely due to an inability for HER3DB1DB2 to function as a co-transcription factor. To date, various functions of nuclear localized HER family receptors have been identified. The present study is the first to map specific TADs on a nuclear HER family member, and further TAD mapping studies of both HER2 and EGFR are underway.