Cellular housekeeping. Examples of challenges in housekeeping gene detection. (A) Genes having several splice variants could have different expression levels [indicated by the number of reads (black bars)] for different parts of the gene. (B) Duplicative regions, due to pseudogenes and other duplications, complicate unique read alignments, thus biasing expression-level measurement. (C) Expression measurement has several biases, including the lower expression (on average) of the upstream exons due to imperfect reverse transcription resulting in partial cDNA molecules. Not altered. CC. Human housekeeping genes, revisited Eli Eisenberg, Erez Y. Levanon