CD200 and hydrops fetalis: Immunofluorescence staining was employed to identify the HF progenitor cells, characterized as basal keratinocytes in the lower segment of the HFs, which were strongly positive for CD34 and either negative or weakly positive for CD200, as depicted in Figure 1C. We then selected progenitor cell areas as ROIs for transcriptional profiling, designating one ROI per HF.