Breast cancer is known to be one of the most aggressive types of cancer in women, with an alarming estimate of 300,000 cases being diagnosed every year. Early detection of an aggressive and malignant cancer is imperative for receiving targeted therapy and treatment. Detecting the overexpression of the HER-2 protein, a major contributor to breast cancer, can be utilized in early stages to reduce the risk of the cancer spreading and fatal outcomes. This project aims to design a cell-free biosensor that detects HER-2 overexpression using a DNA aptamer with high specificity for the protein. Once the HER-2 aptamer and the protein bind, a toehold switch is then activated, producing a reporter signal that indicates HER-2 overexpression. With further experimental verification, the final design for the cell-free biosensor and research suggests that this approach offers a low-cost alternative for detecting HER-2 overexpression and improving the accessibility of cancer detection methods.