CD44 and breast cancer: CD44, a nonkinase transmembrane glycoprotein, is thought to play a role in CSCs.22 As mentioned previously, CD44 was first used as a CSC marker in breast cancer.4 Then CD44 was identified as a CSC marker in colorectal cancer (CRC),23,24 pancreatic cancer,25 OC,26 gastric cancer,27 and others.