A compound of Formula Bf: or a pharmaceutically acceptable form thereof, wherein: X 1 is CR 1; X 3 is selected from N and CR 4; R 1 is ester, which is substituted with 0, 1, 2, or 3 R 12; R 3, and R 4 are each independently selected from H, alkyl, alkoxy, halo, CN, and NO 2, each of which is substituted with 0, 1, 2, or 3 R 12; R 5 is selected from H, alkyl, alkenyl, alkynyl, -NR 10 R 11, -OR 11, and -SR 11, each of which is independently substituted with 0, 1, 2, or 3 R 12; or when R 5 is -NR 10 R 11, then R 10 and R 11 can be taken together with the nitrogen atom to which they are attached to form a heterocyclyl or heteroaryl group, each of which is substituted with 0, 1, 2, or 3 R 12; R 4 and R 5 can be taken together with the carbon atoms to which they are attached to form a cycloalkyl, heterocyclyl, aryl, or heteroaryl group, each of which is independently substituted with 0, 1, 2, or 3 R 12; R 8 is selected from H, acyl, alkyl, cycloalkyl, amido, amino, carbamate, carbonyl, and urea, each of which is substituted with 0, 1, 2, or 3 R 12; each R 10 and R 11 are independently selected from H, acyl, alkyl, carbonyl, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl, each of which is independently substituted with 0, 1, 2, or 3 R 12; and each R 12 is independently selected from acyl, alkyl, alkenyl, alkynyl, alkoxy, aryloxy, alkoxycarbonyl, amido, amino, carbonate, carbamate, carbonyl, ester, halo, CN, NO 2, hydroxyl, phosphate, phosphonate, phosphinate, phosphine oxide, thio, alkylthio, arylthio, thiocarbonyl, sulfonyl, sulfonamidyl, sulfoxyl, sulfonate, urea, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl for use in a method for treating cancer associated with one or more insertion or deletion mutations in the exon 20 domain of EGFR or HER2, the method comprising administering to a subject in need thereof, a therapeutically effective amount of said compound of Formula Bf or a pharmaceutically acceptable salt thereof. X 1 is CR 1; X 3 is selected from N and CR 4; R 1 is ester, which is substituted with 0, 1, 2, or 3 R 12; R 3, and R 4 are each independently selected from H, alkyl, alkoxy, halo, CN, and NO 2, each of which is substituted with 0, 1, 2, or 3 R 12; R 5 is selected from H, alkyl, alkenyl, alkynyl, -NR 10 R 11, -OR 11, and -SR 11, each of which is independently substituted with 0, 1, 2, or 3 R 12; or when R 5 is -NR 10 R 11, then R 10 and R 11 can be taken together with the nitrogen atom to which they are attached to form a heterocyclyl or heteroaryl group, each of which is substituted with 0, 1, 2, or 3 R 12; R 4 and R 5 can be taken together with the carbon atoms to which they are attached to form a cycloalkyl, heterocyclyl, aryl, or heteroaryl group, each of which is independently substituted with 0, 1, 2, or 3 R 12; R 8 is selected from H, acyl, alkyl, cycloalkyl, amido, amino, carbamate, carbonyl, and urea, each of which is substituted with 0, 1, 2, or 3 R 12; each R 10 and R 11 are independently selected from H, acyl, alkyl, carbonyl, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl, each of which is independently substituted with 0, 1, 2, or 3 R 12; and each R 12 is independently selected from acyl, alkyl, alkenyl, alkynyl, alkoxy, aryloxy, alkoxycarbonyl, amido, amino, carbonate, carbamate, carbonyl, ester, halo, CN, NO 2, hydroxyl, phosphate, phosphonate, phosphinate, phosphine oxide, thio, alkylthio, arylthio, thiocarbonyl, sulfonyl, sulfonamidyl, sulfoxyl, sulfonate, urea, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl for use in a method for treating cancer associated with one or more insertion or deletion mutations in the exon 20 domain of EGFR or HER2, the method comprising administering to a subject in need thereof, a therapeutically effective amount of said compound of Formula Bf or a pharmaceutically acceptable salt thereof.