Phase I - Topic 307 Breast cancer is the most common cancer in women. The major complication after breast conserving surgery (BCS) is local recurrence of the cancer. The key factor in reducing local recurrence is obtaining a: clean"""""""" margin of normal tissue around the removed lump. This is currently assessed via pathology that takes 3-5 days and analyzes only about 1/10 of 1% of the specimen. If tumor cells are identified within ~2mm of the tissue edge, the patient is called back for additional surgery. Currently, this happens ~40% of the time, creating an enormous burden in time, cost and patient anxiety. Even worse, local recurrence happens in up to 20% of patients with apparently clean margins, indicating that tumor cells are often missed during sampling. This proposal outlines plans to establish parameters for the use of novel fluorescent protease sensors to rapidly detect tumor margins in excised breast tissues. Since these probes can be topically sprayed onto tissues, they allow detection of residual tumor cells throughout the entire sample. Furthermore, because they activate rapidly, they can be used to guide surgery. Thus, the probes described here have the potential to both reduce the need for multiple surgeries and improve overall patient outcome.