A protein kinase inhibitor of the Formula I or a tautomer, a stereoisomer or a mixture of stereoisomers, or a pharmaceutically acceptable salt, solvate or hydrate thereof, wherein: X 1 is N or CR t 1; X 2 is N or CR t 2; X 3 is N or CR t 3; X 4 is N or CH; wherein X 1, X 2, X 3, and X 4 are independently selected; R t 1 is selected from -H, halo, -COOH, -CN, -CH 2 OH, C 1 -C 4 -alkyl, -O(C 1 -C 3 -alkyl); R t 2 is selected from -H, halo, -CH 3, -CH 2 CH 3, -OH, -OCH 3, and -NH 2; R t 3 is selected from -H, halo, -S r R 4, -CN, and C YR 4; ring A is aryl or a 5- or 6-membered heteroaryl ring, wherein the heteroaryl forming ring A contains 1-2 heteroatoms selected from N, S, and O, and wherein ring A is optionally substituted by 1-4 R a groups; ring B is a phenyl or 5- or 6-membered heteroaryl ring, wherein the heteroaryl forming ring B contains 1-2 ring heteroatoms selected from N or S and wherein ring B is optionally substituted by 1-5 R b groups; R a and R b are each independently selected from -H, halo, -CN, -R 6, -OR 4, -NR 4 R 5, -C YR 4, -S r R 4, -SO 2 NR 4 R 5, -NR 4 SO 2 NR 4 R 5; alternatively, one R b substituent of ring B may be ring C, wherein ring C is a 5- or 6-membered heteroaryl or heterocyclyl ring comprising carbon atoms and 1-3 heteroatoms independently selected from O, N and S r, and wherein ring C is optionally substituted by 1 to 5 substituents R c; each R c is independently selected from -H, halo, and -R 6; alternatively one of the R b substituents may have a structure -L 2 -D, wherein D is a ring, L 2 is (CH 2) z, and z is 1, 2, 3 or 4; or L 2 is (OCH 2) x, wherein x is 0, 1, 2 or 3, and wherein ring D is a 5- or 6-membered heteroaryl or heterocyclyl ring comprising carbon atoms and 1-3 heteroatoms independently selected from O, N and S r, and wherein ring D is optionally substituted by 1 to 5 substituents R d; each R d is independently selected from -H, halo, R 6, -OR 4 or -NR 4 R 5; L 1 represents a NR 3 C or C NR 3; each Y is independently selected from a chemical bond, -O-, -S-, and -NR 5; each R 3, each R 4 and each R 5 is independently selected from H and C 1 -C 6 -alkyl, wherein alternatively an NR 4 R 5 group may represent a 5- or 6-membered saturated, or unsaturated ring; each R 6 is independently selected from C 1 -C 6 -alkyl or C 2 -C 6 -alkenyl; and r is 0, 1, or 2. X 1 is N or CR t 1; X 2 is N or CR t 2; X 3 is N or CR t 3; X 4 is N or CH; wherein X 1, X 2, X 3, and X 4 are independently selected; R t 1 is selected from -H, halo, -COOH, -CN, -CH 2 OH, C 1 -C 4 -alkyl, -O(C 1 -C 3 -alkyl); R t 2 is selected from -H, halo, -CH 3, -CH 2 CH 3, -OH, -OCH 3, and -NH 2; R t 3 is selected from -H, halo, -S r R 4, -CN, and C YR 4; ring A is aryl or a 5- or 6-membered heteroaryl ring, wherein the heteroaryl forming ring A contains 1-2 heteroatoms selected from N, S, and O, and wherein ring A is optionally substituted by 1-4 R a groups; ring B is a phenyl or 5- or 6-membered heteroaryl ring, wherein the heteroaryl forming ring B contains 1-2 ring heteroatoms selected from N or S and wherein ring B is optionally substituted by 1-5 R b groups; R a and R b are each independently selected from -H, halo, -CN, -R 6, -OR 4, -NR 4 R 5, -C YR 4, -S r R 4, -SO 2 NR 4 R 5, -NR 4 SO 2 NR 4 R 5; alternatively, one R b substituent of ring B may be ring C, wherein ring C is a 5- or 6-membered heteroaryl or heterocyclyl ring comprising carbon atoms and 1-3 heteroatoms independently selected from O, N and S r, and wherein ring C is optionally substituted by 1 to 5 substituents R c; each R c is independently selected from -H, halo, and -R 6; alternatively one of the R b substituents may have a structure -L 2 -D, wherein D is a ring, L 2 is (CH 2) z, and z is 1, 2, 3 or 4; or L 2 is (OCH 2) x, wherein x is 0, 1, 2 or 3, and wherein ring D is a 5- or 6-membered heteroaryl or heterocyclyl ring comprising carbon atoms and 1-3 heteroatoms independently selected from O, N and S r, and wherein ring D is optionally substituted by 1 to 5 substituents R d; each R d is independently selected from -H, halo, R 6, -OR 4 or -NR 4 R 5; L 1 represents a NR 3 C or C NR 3; each Y is independently selected from a chemical bond, -O-, -S-, and -NR 5; each R 3, each R 4 and each R 5 is independently selected from H and C 1 -C 6 -alkyl, wherein alternatively an NR 4 R 5 group may represent a 5- or 6-membered saturated, or unsaturated ring; each R 6 is independently selected from C 1 -C 6 -alkyl or C 2 -C 6 -alkenyl; and r is 0, 1, or 2.