AbstractSignal transducer and activator of transcription 5 (STAT5a and STAT5b) are intrinsically critical for normal hematopoiesis but are also expressed in stromal cells. Here, STAT5ab knockout (KO) was generated with a variety of bone marrow hematopoietic and stromal Cre transgenic mouse strains. Vav1-Cre, the positive control for loss of multipotent hematopoietic function, surprisingly dysregulated niche factor mRNA expression and deleted STAT5ab in CD45neg cells. Single cell transcriptome analysis of bone marrow from wild-type or Vav1-Cre KO mice showed hematopoietic stem cell myeloid commitment priming and upregulated protein translation genes. Nes+ cells were detected in both CD45neg and CD45+ clusters and deletion of STAT5ab with Nes-Cre caused hematopoietic repopulating defects. To follow up on these promiscuous Cre promoter deletions in CD45neg and CD45+ bone marrow cell populations, more stroma-specific Cre strains were generated and demonstrated reduction in multipotent hematopoietic progenitors. Functional support for niche-supporting activity was assessed using STAT5-deficient MSCs. With Lepr-Cre, niche factor mRNAs were downregulated by STAT5ab deletion with validation of reduced IGF-1 and CXCL12 proteins. Furthermore, computational analyses (differential expression/co-expression) revealed a key role for STAT5ab/Cish balance with Cish strongly co-expressed in MSCs and HSCs primed for differentiation. Therefore STAT5ab-associated gene regulation supports the bone marrow microenvironment.