Precision medicine, a medical modality focusing on tailoring medical decision-making to individual patients, is changing the way we think about, prevent, treat, and monitor many diseases, including those requiring gene and cell therapies. Both gene and cell therapies involve the therapeutic transfer of new genetic material into a target cell with the goal of treating disease. The fields of gene and cell therapies are growing, but there are many unknowns and reasons to be cautious remain. Selecting the right patient for the right therapy and monitoring that patient’s response to the therapy is imperative. Biomarkers are tools that can facilitate selection and monitoring of gene and cell therapies, and their proper identification and application allows patients to be treated accurately, effectively, and safely. Several biomarkers of disease, immune, cellular, and molecular responses to gene and cell therapies are available, and the role of biomarkers will expand as gene and cell therapies continue to develop. With the rapid growth of gene and cell therapies, biotechnology and pharmaceutical companies face a call to action: we must establish proper selection and monitoring protocols to provide patients with the safest and most effective therapeutic options for genetic diseases. This article presents two case studies from a biopharmaceutical company’s clinical programs for gene and allogeneic cell therapies and provides a primer for the relevance of precision medicine applications.