Live cancer DU145 cells that stably express GFP (Green Fluorescence Protein)-labeled EphA2 mutants and tumor cells derived from mouse epithelia (i.e., 728 cells), where EphA1/EphA2 genes have been knocked out but with stable expression of GFP-labelled mutant receptors, have been employed to perform FCS (Fluorescence Correlation Spectroscopy) measurements. The gene discussed is EPHA1; the disease is cancer.