PECAM1 and hydrops fetalis: Using the non-HF cohort alone, the eight cell populations were manually annotated using literature-derived annotation using the most enriched marker genes in each population: cardiomyocytes (TTN, MHY6, RYR2, NPPA), fibroblasts (DCN, C7, CFD, ADH1B, APOD), endothelial cells (VWF, IFI27, AQP1, HLA-E, PECAM1), smooth muscle cells (MYH11, LBH, CALD1, LMOD1, MYLK), pericytes (RGS5, AGT, ABCC9, STEAP4, PDFGRB), mesothelial cells (HP, ITLN1, PRG4, PLA2G2A, C3), myeloid cells (CD163, AIF1, MS4A6A, CCL3L3, CCL4L2), and lymphoid cells (CCL5, PTPRC, TRBC2, CD2, CD52) (Figure 1C).