CD44 and cancer: There are several factors that may contribute to the enhanced penetration of DOX/HA-b-PCDA NPs into BCSC-enriched 4T1 mammospheres [78–83]: (1) mammospheres are enriched in CD44 receptors, which can bind to HA on the surface of NPs; (2) HA modificaition improves the uptake of NPs by cancer cells, as HA is a natural extracellular matrix component that signals tissue repair to cancer cells; (3) HA provides specificity towards cancer cells by exploiting their overexpression of CD44 compared to normal cells.