Marc L. Sprouse et al. isolated polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) as well as CTCs and cocultured PMN-MDSCs with CTCs and human brain metastatic breast cancer cells (Luc-MDA-MB-231BR) or melanoma (Luc-70 W-SM3) cells and found that CTCs and tumor cells could form heterotypic clusters with PMN-MDSCs and that PMN-MDSC-cocultured tumor cells exhibited relatively high expression of Ki67. Here, MKI67 is linked to breast cancer.