# Discovery and characterization of LNCSOX17 as an essential regulator in human endoderm formation

Long non-coding RNAs (lncRNAs) have emerged as fundamental regulators in various biological processes, including embryonic development and cellular differentiation. Despite much progress over the past decade, the genome-wide annotation of lncRNAs remains incomplete and many known non-coding loci are still poorly characterized. Here, we report the discovery of a previously not annotated lncRNA that is transcribed upstream of the SOX17 gene and located within the same topologically associating domain. We termed it LNCSOX17 and show that it is induced following SOX17 activation but its expression is more tightly restricted to early definitive endoderm. Loss of LNCSOX17 affects crucial functions independent of SOX17 and leads to an aberrant endodermal transcriptome, signaling pathway deregulation and epithelial to mesenchymal transition defects. Consequently, cells lacking the lncRNA cannot further differentiate into more mature endodermal cell types. We identified and characterized LNCSOX17 as an essential new actor in early human endoderm, thereby further expanding the list of functionally important non-coding regulators.