Gene Regulation in Eukaryotes The latest estimates are that a human cell, a eukaryotic cell, contains some 21,000 genes. Some of these are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells. Some are expressed as a cell enters a particular pathway of differentiation. Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the genes for the antibody it synthesizes. Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.