Homo sapiens longevity assurance homologue 2 of yeast LAG1 (LASS2)/tumor metastasis suppressor gene 1 (TMSG1) was a novel tumor metastasis-related gene identified using messenger RNA differential display from non-metastatic human prostate cancer cell variants. The mechanism of LASS2/TMSG1 inhibiting tumor invasion metastasis in breast cancer cells had not been well investigated. In the present study, a full length of 1.2 kb LASS2/TMSG1 complementary DNA (cDNA) coding for a protein of 380 amino acids was cloned. PcDNA3 eukaryotic expression plasmids of LASS2/TMSG1 were constructed and transfected into human breast cancer cell line MCF-7 by lipofectin transfection method. And, the biological effects were observed comparing with control groups. As the result, LASS2/TMSG1 inhibited cell growth in vitro by increasing apoptosis and changing cell cycle distribution. Furthermore, the vacuolar ATPase (V-ATPase) activity and extracellular hydrogen ion concentration were significantly decreased and the activity of secreted matrix metalloproteinase-2 (MMP-2) was downregulated in MCF-7 cells overexpressing LASS2/TMSG1 compared with the controls. Therefore, LASS2/TMSG1 may inhibit growth and invasion of breast cancer cell in vitro through decreasing V-ATPase activity and extracellular hydrogen ion concentration and inactivating secreted MMP-2. The findings provided the evidence that the LASS2/TMSG1 gene had tumor growth and invasion suppressor function in human breast cancer cell and may provide a promising target for cancer metastasis diagnosis and therapy.