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 [[peptide]]s, [[hormone]]s, [[neurotransmitter]]s, [[growth factor]]s, or [[cytokine]]s 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.