It is often assumed mutant frequencies, as measured in a DNA sample, faithfully represent
basic mutation rates associated with these mutations. This paradigm was extremely helpful
for in vitro studies of the mechanisms of mutagenesis/repair and causes of mutations.
However, in vivo, mutant fractions appear to vary dramatically and randomly from sample to
sample. It's unlikely that basic mutational rates vary so much. Such variations are probably
caused by clonal expansions of mutants within tissue. Whether a particular tissue sample …