Genes that are widely expressed in abundance and are usually used as reference genes for normalization in real-time PCR with the assumption of ‘constant expression’. Also called maintenance genes. The current trend is first to check which housekeeping genes are suitable for the target cell or tissue and then to use more than one of them in normalization in qPCR assays. See for EXPOLDB: The most constantly expressed housekeeping genes housekeeping genes showing the least inter-individual difference in their expression levels. Note that not all housekeeping genes are ideal normalizers, and not all normalizers are housekeeping genes. See Dheda, 2004.