Target Organ The site of action of a toxin.  Most toxins are specific in their toxicity and damage only particular tissues or organs. Tetrodotoxin A potent neurotoxin produced in some species of frogs, puffer fish and other invertebrates. Tissue A group of cells with similar structure and function. There are four types of tissues: epithelial tissue, connective tissue, muscle tissue, and nerve tissue. Tumor A general term for an uncontrolled growth of cells that becomes progressively worse with time.  Tumors may be benign or malignant.  Same as neoplasm. Tumor Suppressor Gene Genes present in normal cells that serve to prevent a cell with damaged DNA from proliferating and evolving into an uncontrolled growth.  Sometimes referred to as anti-oncogenes.  The p53 gene is a tumor suppressor gene.