California State University Channel Islands View Institution's Website 1 article published in JoVE Biology A Quantitative Assay for Insulin-expressing Colony-forming Progenitors Michael Winkler1, Nancy Trieu2, Tao Feng2, Liang Jin2, Stephanie Walker2, Lipi Singh2, Hsun Teresa Ku2,3 1Department of Biotechnology & Bioinformatics, California State University Channel Islands, 2Department of Diabetes, Endocrinology and Metabolism, Beckman Research Institute of City of Hope, 3The Irell & Manella Graduate School of Biological Sciences, Beckman Research Institute of City of Hope A three-dimensional clonogenic assay that allows pancreatic-like progenitors to differentiate into insulin-expressing colonies is described. This method takes advantage of semi-solid media containing methylcellulose, Matrigel and growth factors, in which single progenitors proliferate and differentiate in vitro, permitting quantification of the number of functional progenitors in a population.