The present disclosure provides substituted 9H-pyrimido[4,5-b]indoles and 5H-pyrido[4,3-b]indoles and related analogs represented by Formula I: and the pharmaceutically acceptable salts, hydrates, and solvates thereof, wherein R1a, A, B1, B2, G, X1, Y1, Y2, and Y3 are as defined as set forth in the specification. The present disclosure is also directed to the use of compounds of Formula I to treat a condition or disorder responsive to inhibition of BET bromodomains. Compounds of the present disclosure are especially useful for treating cancer.