 CTCs are believed to be causing metastasis and may provide a non-invasive alternative to organ biopsies for the detection, characterization and monitoring of solid cancers. CTC numbers have been shown to be a strong predictor of Progression Free Survival and Overall Survival for mCRC patients. The CellSearch system Veridex LLC, Ratinas, NJ, USA currently is the only FDA approved test for the evaluation of CTC numbers in metastatic breast, prostate and colorectal cancer. However, the rarity of CTCs in the blood leads to limited capture efficiency and the CellSearch system fixes cells, preventing further molecular characterization of CTCs by functional assays and primary cell culture. In this protocol the CellSearch system will be compared to a new technology, called the Flexible Micro Spring Array FMSA device, developed by Dr. Zheng, Department of Bioengineering, Penn State University, University Park. This novel approach enables size-exclusion based filtration for viable CTC enrichment. The FMSA device is inexpensive, works rapidly, and retains viable CTCs for further biological study. Using both the CellSearch system and the FMSA device, we will determine the kinetics of CTC shedding into circulation, develop an effective system for isolation, enumeration, and further enrichment CTCs, and use this system to find characteristics of different CTC populations. 