Normal signaling pathways can malfunction, resulting in abnormal cellular responses. This breakdown in communication can result in problems, including but not limited to developmental defects, Alzheimer's disease, diabetes, and cancer. For example, when RTK signaling breaks down during epidermal growth factor transmission, the mechanisms that regulate the division of cells during mitosis fail to stop the cell at required checkpoints. The cells divide without inhibition, which can result in cancer. For example, about 20% of breast cancer patients have a genetic mutation that leads to excessive numbers of the RTK known as HER2. This RTK triggers rapid cell division, leading to a particularly aggressive form of cancer. One way to combat cancers caused by RTK receptors is to use medications that block the receptor binding site or inhibit phosphorylation on the cytoplasmic side of the receptor. Indeed, research has found that using chemotherapy that blocks the functioning of HER2 receptors leads to fewer cell divisions and a slower progression of cancer.
As G-protein-coupled receptors are the most common type of cell surface transmembrane receptor, there are many cellular functions that can be affected when one malfunctions. Sometimes, the G proteins themselves are the cause of the problem. If a protein itself is defective, it is unable to be activated by the guanosine triphosphate (GTP) when it attempts to bind with the protein. Symptoms associated with the human diseases whooping cough (pertussis) and cholera are related to damaged G proteins, as are the symptoms of botulism, a condition caused by toxins from the bacterium known as Clostridium botulinum. Medications to alleviate the symptoms associated with pertussis, cholera, or botulism work by targeting the G protein pathways.