The gene product of spleen tyrosine kinase (SYK) has been implicated in the suppression of breast cancer invasion. We previously reported that SYK expression is lost in a subset of breast cancer; primarily by methylation-mediated gene silencing. In our study, we explored the possibility of using a DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (AZA), to suppress breast cancer cell invasion by restoring SYK expression. We found that AZA treatment reestablished the expression of SYK(L) that was accompanied by suppression of the invasion capacity of SYK-negative cells. This invasion inhibition was not due to global cellular toxicity since this treatment did not affect overall cell proliferation. This decreased invasiveness by AZA treatment was diminished by piceatannol, a SYK inhibitor, suggesting that SYK play a significant role in AZA-inducible invasion suppression. SYK promoter hypermethylation was found infrequent in pathologically normal mammary tissues or benign lesions (<5%). In contrast, SYK methylation was frequently identified in ductal carcinoma in situ ( approximately 45%) and invasive ductal carcinoma (47% in node-negative and 40% in node-positive cases), indicating that the hypermethylation of SYK occurs at a stage prior to the development of invasion phenotypes. All these results suggested a potential use of SYK methylation as a valuable biomarker to detect early cancerous lesions and support the use of AZA as a new reagent to the management of advanced breast cancer.