A method and system for identifying a defect or contamination on a surface of a sample. In addition, the invention includes mutants of these three genes, as well as methods for utilizing these genes, and their encoded polypeptides, in diagnosing and treating abnormal cell proliferation. If no such value is stored, additional steps are provided to generate a pilot tone for input to the modulator and to recognize, from the signal content of a modulated optical signal output by the modulator, operation of the modulator at the required bias point, adjusting the value of bias voltage being applied to the modulator as required.