In the current study, though melatonin showed weak cytotoxicity in MDA-MB-231 breast cancer cells, it effectively attenuated the expression of 45S pre-rRNA, UBF, and fibrillarin at mRNA level as well as downregulated nucleolar proteins such as UBF and fibrillarin at protein level in MDA-MB-231 cells. This evidence concerns the gene FBL and breast cancer.