MS4A3 regulates colony formation and TKI resistance of BCR-ABL1⁺ cells ex vivo, and BCR-ABL1⁺ cell engraftment in vivo. (A-D) Effects of MS4A3 KD or ectopic MS4A3 expression on colony formation of CD34⁺ cells of CML samples (A-B) and CB samples (C-D). Colony formation: number of GM-colony formed after culture for 14 days. (E) Design of LTC-IC and xenograft assays. (F) Dox-shMS4A3-GFP⁺ cells were cultured ±0.1 μg/mL dox in 6-week LTC-IC cultures, then plated in colony assays. Colonies were genotyped by FISH (representative images shown). IM^-dox^- reference colony count average: 248. (G) Dox-shMS4A3-GFP⁺ cells were injected into NSG mice that were kept on dox water or normal water. hCD45⁺GFP⁺ cells in the BM (left), PB (middle), and spleen (right) were quantified with flow cytometry at 12 weeks postinjection. (H) BCR-ABL1 expression in the BM (left), PB (middle), and spleen (right) of recipient mice were detected by digital PCR at 12 weeks postinjection.