A hearing aid includes a microphone for providing an electrical signal representative of a received acoustic signal, a means for filtering the electrical signal and a means for automatic switching. In addition, the present invention provides compositions and methods useful in the diagnosis and treatment of conditions associated with skin pigmentation defects, including but not limited to freckling, as well as endocrine tumors including, but not limited to adrenal and pituitary tumors. The cell line derived from a transgenic mammal comprises: a conditional oncogene, transforming gene or immortalising gene or a cell cycle affecting gene; and a cell type specific promoter. The isoprene produced in such a cultured medium can then be recovered and polymerized into synthetic rubbers and other useful polymeric materials. The second and third clocks are generated to isolate the output node from inputs to the first dynamic logic circuit responsive to the scan mode signal indicating that scan is active.