Islet-1, Insulin gene enhancer protein ISL-1, is a protein that in humans is encoded by the ISL1 gene. This gene encodes a transcription factor containing two N-terminal LIM domains and one C-terminal homeo domain. The encoded protein plays an important role in the embryogenesis of pancreatic islets of Langerhans. ISL1 has been shown to interact with Estrogen Receptor alpha.

Islet-1 produces a strong nuclear staining in the islets of normal pancreas and tumor cells of the pancreatic neuroendocrine tumors. Islet-1 has been found to be a reliable marker of pancreatic endocrine tumors and metastasis. It shows a comparable sensitivity and specificity as CDX2 as a marker of ileal and appendiceal neuroendocrine tumors and their metastasis. TTF1 is very rarely expressed in well-differentiated gastroentero-pancreatic endocrine tumors. Therefore, the panel of Islet-1 CDX2, and TTF1 and they may be useful for examining metastasis of well-differentiated endocrine carcinomas of unknown origin.