A PLK4 antagonist for use in a method of treating cancer in a patient with cancer, wherein said cancer is characterized by a PTEN gene mutation, wherein the PTEN gene mutation results in reduced expression of PTEN as compared to a normal control, expression of a non-functional or limited functioning PTEN protein, or a loss of PTEN expression, wherein the PLK4 antagonist is represented by structural formula: or a pharmaceutically acceptable salt thereof, wherein R a is halogen, cyano, -NR 1 R 2, -NR 2 C R 1, -C OR 1, -OC R 1, -N(R 2)C NR 1 R 2, -OR 1 or C 1-6 alkyl, wherein the C 1-6 alkyl is optionally substituted with one or more substituents selected from the group consisting of halogen, -OH, -SH, -O(C 1-6 alkyl), -S(C 1-6 alkyl) and C 1-6 haloalkoxy; each R 1 is independently -H or C 1-6 alkyl, wherein the C 1-6 alkyl is optionally substituted with one or more substituents independently selected from the group consisting of halogen, -OH, -SH, -O(C 1-3 alkyl), -S(C 1-3 alkyl) and C 1-6 haloalkoxy; each R 2 independently is -H or C 1-6 alkyl, or, taken together with NR 1, forms a non-aromatic heterocyclic group optionally substituted with one or more substituents selected from the group consisting of =O, =S, halogen, nitro, cyano, hydroxy, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 hydroxyalkyl, amino, C 1-6 alkylamino, C 1-6 dialkylamino, C 1-6 aminoalkyl, (C 1-6 alkylamino)C 1-6 alkyl, (C 1-6 dialkylamino)C 1-6 alkyl, (phenyl)C 1-6 alkyl, (5-6 membered heteroaryl)C 1-6 alkyl, C 1-6 alkoxy, C 1-6 haloalkoxy, C 1-6 alkylcarbonyloxy, C 1-6 alkoxycarbonyl, C 1-6 alkylcarbonyl, phenyl and 5-6 membered heteroaryl; R 4 is -H, C 1 -C 6 alkyl, phenyl, -C (C 1-6 alkyl), -C (phenyl), -C O(C 1-6 alkyl), -C O(phenyl), -S 2 (C 1-6 alkyl) or -S 2 (phenyl); and R 6 is optionally substituted phenyl, optionally substituted 5-12 membered heteroaryl, -CH 2 -(optionally substituted phenyl), -CH 2 -(optionally substituted 5-12 membered heteroaryl), -CH 2 -CH 2 -(optionally substituted phenyl), -CH 2 -CH 2 -(optionally substituted 5-12 membered heteroaryl), -CH=CH-(optionally substituted phenyl), -CH=CH-(optionally substituted 5-12 membered heteroaryl), -C≡C-(optionally substituted phenyl) or -C≡C-(optionally substituted 5-12 membered heteroaryl. R a is halogen, cyano, -NR 1 R 2, -NR 2 C R 1, -C OR 1, -OC R 1, -N(R 2)C NR 1 R 2, -OR 1 or C 1-6 alkyl, wherein the C 1-6 alkyl is optionally substituted with one or more substituents selected from the group consisting of halogen, -OH, -SH, -O(C 1-6 alkyl), -S(C 1-6 alkyl) and C 1-6 haloalkoxy; each R 1 is independently -H or C 1-6 alkyl, wherein the C 1-6 alkyl is optionally substituted with one or more substituents independently selected from the group consisting of halogen, -OH, -SH, -O(C 1-3 alkyl), -S(C 1-3 alkyl) and C 1-6 haloalkoxy; each R 2 independently is -H or C 1-6 alkyl, or, taken together with NR 1, forms a non-aromatic heterocyclic group optionally substituted with one or more substituents selected from the group consisting of =O, =S, halogen, nitro, cyano, hydroxy, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 hydroxyalkyl, amino, C 1-6 alkylamino, C 1-6 dialkylamino, C 1-6 aminoalkyl, (C 1-6 alkylamino)C 1-6 alkyl, (C 1-6 dialkylamino)C 1-6 alkyl, (phenyl)C 1-6 alkyl, (5-6 membered heteroaryl)C 1-6 alkyl, C 1-6 alkoxy, C 1-6 haloalkoxy, C 1-6 alkylcarbonyloxy, C 1-6 alkoxycarbonyl, C 1-6 alkylcarbonyl, phenyl and 5-6 membered heteroaryl; R 4 is -H, C 1 -C 6 alkyl, phenyl, -C (C 1-6 alkyl), -C (phenyl), -C O(C 1-6 alkyl), -C O(phenyl), -S 2 (C 1-6 alkyl) or -S 2 (phenyl); and