ATP-binding cassette (ABC) transporters are a large family of proteins that use the energy from ATP hydrolysis to transport various molecules across cellular membranes. They play crucial roles in the movement of substances such as ions, lipids, and drugs, helping maintain cellular homeostasis and influencing drug resistance in some cancer cells. These transporters are characterized by their conserved ABC domain, which binds and hydrolyzes ATP. ABC transporters are found in all living organisms, from bacteria to humans, and are essential for processes like nutrient uptake and waste removal, highlighting their importance in cellular function and health.