Therefore, given (1) the demonstrated increase in anti-tumor efficacy of chrono-chemotherapy, (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), we sought to determine whether optimal timing of chemotherapy administration increases anti-tumor efficacy in a model of BMBC. Here, PGP is linked to neoplasm.