Genes are the basic units of heredity that encode proteins. Gene therapy aims to treat diseases by correcting defective genes. There are several approaches, including inserting a normal gene to replace a faulty one. Gene therapy can be somatic, only affecting the individual, or germline, making the effects heritable. Vectors like viruses are used to deliver therapeutic genes to target cells. Common types are adenoviruses, retroviruses, and AAVs. Gene therapy holds promise for treating many genetic disorders and diseases like cancer.