FISH and immunostaining results on identical duodenal sections of 2 female patients who underwent allogeneic, non–T-cell–depleted bone marrow transplantation from male sibling donors. The epithelial basal membrane appears with a broken line in all the images. (Ai) In the epithelium, a cell with a red signal (white arrow) corresponds to Y-chromosome labeling (FISH; original magnification, × 630). X chromosomes are stained green. (Aii) The same cell (white arrow) is not stained with the anticytokeratin antibody (indirect immunofluorescence; original magnification, × 630). Epithelial cells are stained white on a blue background (open black arrowhead). (Aiii) The same cell (white arrow) is stained with the anti-CD45 antibody (indirect immunoperoxidase; original magnification, × 630). A lymphoid cell is stained brown in the lamina propria (white arrowhead). (Aiv) Higher magnification of panel Ai centered on the XY cell in the epithelium (FISH; original magnification, × 3150). X chromosome, green; Y chromosome, red. (Bi) In the epithelium, a cell with a red signal (white arrow) corresponds to Y-chromosome labeling (FISH; original magnification, × 630). X chromosomes are stained green. (Bii) The same cell (white arrow) is not stained with the anticytokeratin antibody (indirect immunofluorescence; original magnification, × 630). (Biii) The same cell (white arrow) is not stained with the anti-CD45 antibody (indirect immunoperoxidase; original magnification, × 630). (Biv) Higher magnification of panel Bi centered on the XY cell in the epithelium (FISH; original magnification, × 3150). X chromosome, green; Y chromosome, red.