I have come across two set of definitions which are not contradictory but different. From wikipedia: A constitutive gene is a gene that is transcribed continually as opposed to a facultative gene, which is only transcribed when needed. A housekeeping gene is typically a constitutive gene that is transcribed at a relatively constant level. The housekeeping gene's products are typically needed for maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions. Examples include actin, GAPDH and ubiquitin. My question from these definitions: What does 'transcribed at a relatively constant level' mean? Does it mean that a constant level of gene product is maintained in a cell? I can't understand there meanings atal. Again,according to Oxford dictionaries Constitutive Genes are genes that are expressed following interaction between a promoter and RNA polymerase without additional regulation. (Oxford Dictionary of Biochemistry and Molecular Biology) Housekeeping genes - Constitutive loci that are theoretically expressed in all cells in order to provide the maintenance activities required by all cells: e.g., genes coding for enzymes of glycolysis and the and the citric acid cycle.(Dictionary of Genetics) These definitions do not tell whether they are continually transcribed or the synthesised products are present in constant amount. Which one best define them?