Pseudogenes are designated with the Greek letter psi, e.g., ΨHBBP1 is a nonfunctional version of the gene encoding the beta chain of human hemoglobin.In fact, most pseudogenes are related to a functioning "parent" gene with which they share much sequence homology.
The human genome contains some 19,000 pseudogenes.That is about the same as the number of functional genes we possess.
Perhaps they are simply an historical accident and represent another category of "junk" DNA.
But perhaps not.
There is growing evidence that pseudogenes can be transcribed into RNA. This RNA cannot be translated into protein, but can provide sequences to which microRNAs (miRNAs) can bind.
MicroRNAs bind to mRNAs inhibiting their translation and/or accelerating their degradation. In either case, they suppress gene expression; that is, the synthesis of protein. With pseudogenes competing for the same miRNAs as their functional "parent", the repression of the functional gene is relieved.
If the functional gene is a proto-oncogene, its elevated expression can lead to cancer. So such a pseudogene represents another category of oncogene.
There is evidence in both mice and humans of cancers that are driven by the elevated expression of a pseudogene related to a known proto-oncogene.