PGP and neoplasm: However, despite many malignancies having a high initial sensitivity to MTAs, several mechanisms can result in drug resistance, including tumor overexpression of P-glycoprotein (Pgp), elevated levels of ß-tubulin subtype III, reduced levels of BRCA1 (the cancer susceptibility gene), elevated levels of the cell cycle inhibitory protein p21, and acquired mutations in ß-tubulin [4–9].