Chem. Sci., 2025, Advance Article DOI: 10.1039/D5SC04171F, Edge Article Xianzhe Wu, Qingxi Ma, Haoran Chen, Chuhao Cheng, Jiapu Li, Feifei Xu, Yun Chen Development of a single-time-point stable isotope probing-mass spectrometry (SIP-MS) assay for revealing enzyme activity heterogeneity in single cells.