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. <EOS>