sp;</span>mutants <span  title="" class="an ">&ensp;</span>using <span  title="" class="an ">&ensp;</span>wild <span  title="" class="an ">&ensp;</span>type <span  title="" class="an ">&ensp;</span>littermates <span  title="" class="an ">&ensp;</span>as <span  title="" class="an ">&ensp;</span>controls <span  title="" class="an ">&ensp;</span>The <span  title="" class="an ">&ensp;</span>null <span  title="" class="an ">&ensp;</span>mutation <span  title="" class="an ">&ensp;</span>confers <span  title="" cl