Previous studies have shown that upon antigen stimulation, the T cell receptor (TCR) undergoes phase separation with the kinase Lck to mediate rapid T cell activation. However, the manner in which costimulating receptors regulate this process has remained unclear. In a study published in PNAS on October 21, researchers from the Institute of Biophysics of the Chinese Academy of Sciences, Zhejiang University, and Harbin Medical University Cancer Hospital, have systematically elucidated how.