Compounds with structures close to those of normal nucleosides or nucl
eobases may be incorporated into cells and then become constituents of
their DNA. Proliferation of such cells could yield mutants. In this a
rticle, the current status of studies on such nucleoside and nucleobas
e analogs is described. Base mispairing mechanisms for these analogs a
re discussed in light of recent biochemical and biophysical findings.