Gating strategies and ATAC-seq results for young and old HSC transplantations. (A) Gating strategy for BM analyses in young (Y, 3 mo old) and old (O, 24 mo old) mice. MPP2 and MPP3, myeloid-biased MPP; MPP4, lymphoid-biased MPP; GMP, granulocyte/macrophage progenitor. (B) Preparation and gating strategy for endosteal BM niche populations. EC, endothelial cell. (C) Gating strategy for peripheral blood chimerism analyses. Results are shown at 4 mo after transplantation. My, myeloid; B, B cells; T, T cells. (D) Top 20 pathways enriched in differentially accessible peaks at promoters (±1 kb from TSS) in post-transplantation vs. pre-transplantation young (left) and old (right) HSC samples. Gray indicates common pathways and black/red refers to pathways specific to each population.