To date, mechanisms reconciling these diametrically opposing phenotypes regulated by CDX2 have not been identified. kinase inhibitors Homeodomain-less isoforms of other homeodomain proteins regulate the expression, localization, or activation of their associated wild type isoform. These truncated variants function as dominant negative isoforms inhibiting transcription by sequestering full-length proteins in the cytoplasm and preventing transcriptional activation of downstream targets. Moreover, they interact indirectly with DNA by binding other cofactors or transcription factors activating distinct subsets of downstream targets. Thus, we explored the possibility that the paradoxical tumor suppressor and oncogenic phenotypes induced by CDX2 might reflect the generation of a previously unknown homeodomain-less variant. Indeed, sequencing of full-length CDX2 transcripts isolated from normal colonic mucosa, adjacent tumors, and colorectal cancer-derived cell lines revealed two distinct transcripts with CDX2/AS containing a frameshiftinduced truncation of the homeobox domain rendering it free of transcriptional activity. The role of CDX2/AS as a dominantnegative isoform of CDX2 was explored in artificial and physiologic cell systems employing exogenous and endogenous targets of CDX2. In contrast to homeodomain-less isoforms of other homeodomain proteins, CDX2/AS did not regulate the transcriptional activity of CDX2. This result was unexpected as both proteins co-localized in the nucleus and exhibit limited colocalization providing opportunity for functional interaction. Given the many genes regulated by CDX2, CDX2/AS could regulate the expression of others not evaluated here. Similarly, CDX2/AS might induce CDX2 to regulate a previously uncharacterized set of genes that may alter cellular phenotypes. These possibilities remain to be explored. In an attempt to determine the function of CDX2/AS, we focused on the localization of CDX2/AS to punctuate nuclear foci. Many proteins localize to distinct nuclear foci such as speckles, paraspeckles, nucleoli, Cajal bodies, GEMS, and PML bodies when observed by indirect immunofluorescence microscopy. Given the enrichment in arginine and serine residues of the carboxy-terminal domain of CDX2/AS, we hypothesized that its localization was most consistent with speckled domains defined by the SR- and SR-like family of splicing factors, which contain domains enriched in arginine and serine residues. Here, ASF/SF2 and SC35 in 293T and RKO colon cancer cells. Nuclear localization of CDX2/AS was lineagesurvival oncogene.