The present invention relates to compounds of formula I that function as inhibitors of BCL6 (B-cell lymphoma 6) activity: Formula I wherein X1, X2, X3, R1, R2, R3, R4 and R5 are each as defined herein. The present invention also relates to processes for the preparation of these compounds, to pharmaceutical compositions comprising them, and to their use in the treatment of proliferative disorders, such as cancer, as well as other diseases or conditions in which BCL6 activity is implicated.