Testis Cancer Definition

The testicles are the male reproductive organs located in the scrotum, just beneath the penis.  The testicles have two main functions:
  1. The testicles produce sperm for reproduction
  2. The testicles make testosterone, the main male hormone responsible for male sexual characteristics

Testis cancer develops when one or more of the cells in the testicle grow out of control.  Normally, hundreds of thousands of cells live, grow and die in an orderly fashion within the testicle.  When one of these cells becomes abnormal and grows uncontrolled, it becomes a cancer.    
The most common cell type in the testicle is called a germ cell.  Normally, the sperm develop from these germ cells.  As germ cells make up approximately 90% of the testicle, the most common type of testis cancer is a germ cell tumor (see Types of Testis Cancer to learn about the rarer testis tumors).