PECAM1 and hereditary elliptocytosis: Immunofluorescence staining of runt-related transcription factor 1 (RUNX1)—another established transcription factor in HE and HSC/HPC development—revealed clusters of positive cells in the outer layer (Extended Data Fig. 6c); on consecutive cryosections the RUNX1+ cluster co-expressed CD31, further corroborating the localization of HE in the outer layer (Extended Data Fig. 6d).