Extended Data Fig. 2: ARX and PDX1 immunostain in human normal islets and PNETs.

a, Double immunofluorescence for PDX1 (red) and ARX (green) in normal islets (marked by dashed white outlines). Scale bar, 50 μm. The results, representing hundreds of islets, verify antibody specificity, lineage-restricted expression and cell distributions: abundant PDX1⁺ β-cells scattered across islets and fewer ARX⁺ α-cells enriched in the islet periphery. b, Top: ARX and PDX1 IHC selectively mark endocrine α- and β-cells, respectively, in normal human islets. Many exocrine and ductal cells also express PDX1, as is well known²⁴. The results represent hundreds of normal islets from multiple individuals, which revealed no ARX⁺ PDX1⁺ DP cells. Thus, although described in rodent embryos²⁴, such cells are absent or extremely rare in the adult human pancreas. Bottom: IHC for ARX in a representative PNET and surrounding normal cells on TMAs from the Dutch cohort. The area boxed in the left image is magnified on the right. ARX⁺ cells dominate in the tumor and mark invasive foci (arrows). c, Range of IHC signal strength in ARX⁺ PNETs (+weak, ++ moderate, +++ strong), contrasted with uniformly robust PDX1 staining. Images are examples selected from 34 ARX⁺ and 31 PDX⁺ cases (Fig. 3b). Scale bars, 50 μm.