$string = 'Furthermore , expression of <GENE> Vpu </GENE> in Jurkat T cells rendered them more susceptible to <GENE> Fas </GENE> - induced death ';
$start = 0;
while (($beg = index($string, '<GENE>', $start)) > -1)
{
     
     $end = index($string, '</GENE>', $start)+7;
     substr($string, $beg, ($end-$beg), '');
     $start = $end;
}
print $string;