General features and critical pathways that contribute to CP CML LSCs being quiescent, refractory to apoptosis, and prone to DNA damage. Typically LSCs represent 1% to 5% of the bulk CML CD34⁺ cells, are enriched by FACS as CD34⁺CD38⁻, and show more variable levels of Ph⁺ cells than bulk CML CD34⁺ cells. Some researchers also include Lin⁻/CD90⁺/CD45RA⁻ cells as part of the CD34⁺CD38⁻ LSC definition.¹¹²  Other FACS approaches can also be used to isolate LSCs by using Hoechst, Pyronin Y, and carboxyfluorescein succinimidyl ester (CFSE) intracellular staining in combination with CD34 to identify quiescent/undivided cells.^39,46,47,147  CD34⁺CD38⁻ CML cells from patients at diagnosis that retain high levels of CFSE (CFSE^max) or are CD34⁺ and both Hoechst^lo and Pyronin Y^lo, and survive exposure to TKI, are often considered surrogate in vitro models for the TKI-resistant cells found in patients with LSC persistence. The schematic diagram of the LSC shows key (but not exhaustive) pathways and components and whether the published evidence points to TKI-dependent (blue) or independent (olive green) mechanisms of regulation. Dotted lines denote translocation of components from the cytoplasm (light red) to the nucleus (white). TK, tyrosine kinase. Activation and repression are denoted according to convention. Specific details of each pathway are described in the text.