A semiconductor memory device includes a memory cell array in which memory cells each having six transistors 11a, 11b, 12a, 12b, 13a and 13b are arranged two-dimensionally on a semiconductor substrate. More particularly, the present invention is directed to hepatitis B variants exhibiting complete or partial resistance to nucleoside analogues andor reduced interactivity with antibodies to viral surface components. A substantially planar butterfly valve is received in the aperture so as to be pivotable between a first position, in which the flow duct is substantially closed and the aperture is substantially open, and a second position, in which the flow duct is substantially open and the aperture is substantially closed. In some holders, the handle is defined by an elongated housing defining an interior volume configured to receive at least a portion of the lever when the jaw members are closed. The change in atomic number alters the doping characteristics of the artificial atoms.