A signaling molecule is a type of molecule that transmits a signal from one cell to another, leading to changes in cellular behavior, gene expression, or metabolism. These signaling molecules can be peptides, hormones, neurotransmitters, growth factors, or cytokines and can play a critical role in physiological processes such as growth and development, homeostasis, and the response to stress or injury. The signal transduction pathways initiated by signaling molecules typically involve the activation of specific receptors on the target cell, leading to changes in the activity of intracellular signaling proteins and ultimately leading to a cellular response.