Selenium (Se) is an important trace element required for maintaining human health. Small amounts of Se has a chemoprotective effect to protect cells from oxidative damage, but higher dosages can introduce damage to DNA, and trigger apoptosis. The effect of Se is dependent on the dosage and chemical form of Se. Previous studies suggest inorganic Se in the form of selenite is more potent than the organic Se form, methylseleninic acid (MSA). The damaging effect of Se has been utilised to combine with chemotherapy drugs to synergistically kill cancer cells. Interestingly, more recent studies suggest that supranutritional dosage of Se can kill tumorous cells while leaving the normal healthy cells unharmed. BRCA1 is a tumour-suppressor gene which functions in sensing and repairing DNA damage. Mutations in BRCA1 are highly associated with breast cancer. Previous epidemiological studies suggest that a patient with BRCA1 mutations appears to be more sensitive to Se supplements. The first objective of this research is to evaluate the interaction of Se dose and chemical form in BRCA1-mutated and non-mutated breast cancer cells in vitro. We applied Se (MSA and selenite) to two commercially-available BRCA1 and two non-BRCA1 mutated breast cancer cell lines. We hypothesised that breast cancer cells with a BRCA1 mutation are more sensitive to DNA damage and death from Se treatment, especially with the inorganic form. Se sensitivity was measured by examining DNA damage and cell viability with the comet assay and the colorimetric MTT assay, respectively. Based on the results from the first objective, the second aim was to examine the combined effect of Se with the chemotherapy drug, cisplatin, and evaluate the synergistic anti-tumour effect on the breast cancer cell lines. The viability after drug exposure was measured with MTT assay. Lastly, we also validated a new technique called Long run real-time PCR (Lord-qPCR) for quantifying DNA damage based on the amplification efficiency in real-time PCR. Se genotoxicity was again tested with this method and the results were compared to the conventional comet assay. Comparison of the comet assays between two BRCA1-mutated breast cell lines (MDA-MB-231 and Sum149pt) and two non-BRCA1-mutated breast cancer cell lines (MCF-7 and MDA-MB-231) showed significantly (p=0.01) greater DNA damage in the cells with BRCA1 mutation for each Se compound. The IC50 values for selenite and MSA, obtained with the MTT assay, were lower in BRCA1-mutated cells than the non-BRCA1-mutated cells. The outcome of the combined treatment with a physiological level of cisplatin and selenite/MSA did not show any synergistic cell inhibition on either BRCA1-mutated or non-mutated breast cancer cell lines. Interestingly, cisplatin at the physiological level, appeared to be less potent than 2 μM of Se treatment. Lastly, Lord-qPCR in comparison to the comet assay is indeed much less labour-intensive. The Se genotoxicity results from Lord-PCR followed the overall trend demonstrated in the comet assay. Surprisingly, DNA lesions predicted from mitochondrial DNA appear to be insignificantly greater than nuclear DNA. However, one drawback of the Lord-qPCR is that the reproducibility is much lower than comet assay, and may therefore require further optimisation.