A number of protein-coding genes are active in all cells and are required for so- called house-keeping functions such as the enzymes of glycolysis, the citric acid cycle and the proteins of the electron transport chain. Furthermore,  some genes are active only in specific  cell kind and are responsible  for defining  the particular  characteristics  and function  of those cells; for instance  immunoglobulin  genes in lymphocytes,  myosin in muscle cells. Additionally,  the proteins  expressed  by any given  cell may changes over time (for instance  during  early development)  or in response  to external  stimuli,  such as hormones. The Eukaryotic cells can regulate the expression of protein-coding genes at a number of levels but a prime site of regulation is transcription.