Breast cancer-resistant protein inhibitor

A breast cancer resistance protein inhibitor containing, as an active ingredient, a flavonoid represented by the following formula (1), (2), (3), (4), or (5):

• (wherein R1 represents a hydrogen atom, a hydroxyl group, a halogen atom, a lower alkoxy group, or a lower alkyl group;
• each of seven R2s, which may be identical to or different from one another, represents a hydrogen atom, a hydroxyl group, a lower alkoxy group, a lower alkyl group, a lower alkenyl group, or a sugar residue, or R1 and the R2 adjacent thereto may together form a pyran ring which may be substituted by a lower alkyl group;
• and R3 represents a hydrogen atom, a hydroxyl group, a halogen atom, a lower alkyl group, a lower alkoxy group, an amino group, or a nitro group);
• (wherein R4 represents a hydrogen atom, a hydroxyl group, a halogen atom, a lower alkoxy group, a lower alkyl group, or a lower alkenyl group;
• each of seven R5s, which may be identical to or different from one another, represents a hydrogen atom, a hydroxyl group, a lower alkoxy group, or a lower alkyl group, or two adjacent R5s may together form a pyran ring which may be substituted by a lower alkyl group;
• and R6 represents a hydrogen atom, a hydroxyl group, a halogen atom, a lower alkoxy group, a lower alkyl group, an amino group, or a nitro group);
• (wherein R7 represents a hydrogen atom, a hydroxyl group, a halogen atom, a lower alkoxy group, or a lower alkyl group;
• each of two R8s, which may be identical to or different from each other, represents a hydrogen atom, a hydroxyl group, a lower alkoxy group, or a lower alkyl group;
• R9 represents a hydrogen atom, a hydroxyl group, a halogen atom, or a lower alkoxy group;
• and each of five R10s, which may be identical to or different from one another, represents a hydrogen atom, a hydroxyl group, or a lower alkoxy group);
• (wherein each of three R11s, which may be identical to or different from one another, represents a hydrogen atom, a hydroxyl group, or a lower alkoxy group);
• or
• (wherein R12 represents a hydrogen atom or a lower alkenyl group;
• R13 represents a hydrogen atom or a hydroxyl group;
• and R14 represents a hydrogen atom), a glycoside of the flavonoid, an ester of the flavonoid, or a salt of the flavonoid.