Scientists at UCSF discovered that the proto-oncogene Src, long thought to be exclusively intracellular, can appear on the exterior membrane of multiple tumor types through autophagolysosomal exocytosis. The team demonstrated extracellular Src (eSrc) on bladder, colorectal, breast and pancreatic cancer cells and showed that antibodies targeting eSrc — including antibody–drug conjugates and immune‑engaging constructs — killed tumor cells and shrank xenografts in mice. The results were published in Science. Lead authors framed eSrc as a broadly expressed, cancer‑associated antigen arising from tumor-specific proteostasis stress and disposal pathways. The finding revives a historically important oncogene as an accessible immunotherapy target and offers a potential route to apply established antibody modalities to tumors previously considered “undruggable” by surface‑directed approaches. Key next steps include validating eSrc prevalence in clinical samples, optimizing antibody formats, and confirming safety given Src’s role in normal tissues when exposed extracellularly.