Phosphoinositide-3-kinase δ (PI3Kδ) is a critical regulator of cell growth and transformation and has been explored as a therapeutic target for a range of diseases. Through the exploration of the thienopyrimidine scaffold, we have identified a ligand-efficient methylation that leads to remarkable selectivity for PI3Kδ over the closely related isoforms. Interrogation through the Free–Wilson analysis highlights the innate selectivity the thienopyrimidine scaffold has for PI3Kδ and provides a predictive model for the activity against the PI3K isoforms.