CD44 and neoplasm: When 4T1 breast cancer cells were grown on PEGDA hydrogels, increasing matrix stiffness from 2.5 kPa to 5.3, 26.1, and 47.1 kPa, resulted in tumor sphere size increasing from 37 to 57 μm, 20 μm, and 12 μm, respectively, and CD44 expression increased from 17-fold to 38-fold, threefold, and twofold, respectively, compared to the baseline levels.