SELL and neoplasm: Compared to the 4T1 system, MDSC expansion was delayed in C57BL/6 mice implanted with AT-3 mammary tumor cells derived from genetically-engineered MMTV-PyMT/B6 transgenic mice (MTAG) (Waight et al., 2013), corresponding with moderate but significant L-selectin downregulation on naïve T cell subsets at ≥21 days post-tumor implantation (Figure 1—figure supplement 3A and B).