Heterogeneity and endocytosis in the established human breast cancer cell line MCF-7 demonstrated with a monoclonal antibody: O. W. Petersen, P. Briand, J. Hilkens, J. Hilgers, and B. van Deurs, Department of Anatomy, The Panum Institute, Copenhagen, Denmark; The Fibiger Laboratory, Copenhagen, Denmark; and Department of Genetics, The Netherlands Cancer Institute, Amsterdam, The Netherlands