PGP and neoplasm: Given (1) the increase in anti-tumor efficacy of chrono-chemotherapy for the treatment of peripheral tumors, (2) the recent revelation of circadian control of blood brain barrier permeability via alterations in efflux transporter function, primarily P-glycoprotein (Pgp) (7, 29, 30), and (3) that one of the most commonly prescribed drugs for the treatment of BMBC, paclitaxel (Taxol), is a substrate for Pgp transporters at the BBB (32–34), the current study sought to determine whether chrono-chemotherapy was a viable treatment strategy to improve the treatment of BMBC.