SIRT3 and breast cancer: Since SIRT3 is known to be present ubiquitously in the body, we speculated that SIRT3 could also be present in the human MDA-MB-231 breast cancer cells in vitro and, in fact, as described in the results section above, we found that deficiency of D-(+)-glucose or L-leucine - but not 4-hydroxytamoxifen - up-regulated the expression of SIRT3 in these cells.