Cancer is a cellular disease, and cells are the basic building blocks of tissue. Cells produce new cells and control the state of cells by receiving instructions that have been encoded. These instructions are called genes. When genes change, normal esophageal cells become cancerous, which is known as a genetic mutation. But this is far from a complete explanation of how and why genetic mutations and cancer cells occur. Further investigation is needed. Esophageal cancer most often occurs in squamous and glandular cells. The epithelial layer of the esophageal wall is rich in squamous cells. Mutations in these cells result in a cancer called squamous cell carcinoma. Cancers that occur in glandular cells are known as adenocarcinomas.