Earlier in this section we discussed that in higher organisms there is considerable proportion of DNA represented by, repetitive sequences. Although it has been speculated that these repetitive sequences might play a regulatory role, their definite function is largely unknown. On the other hand, there may be some repetitive sequences which contain definite genetic information and the most important example of such genes is ribosomal genes which are present in thousands within the same cell for the synthesis of large number of ribosomes needed for protein synthesis. Nevertheless, ribosomal genes make such a small proportion of repetitive sequences, that they alone cannot explain the occurrence of these sequences. In 1980s, it was suggested that these repetitive sequences may not confer any definite advantage to the organisms which contain them. Therefore, sometimes it is said that repetitive sequences represent the DNA which is a repository from which new functional sequences can be obtained. There is, however, no evidence to prove or disprove this hypothesis and it may not be necessary to argue that these DNA sequences, if not of definite advantage to organism, should have been eliminated through natural selection. Actually, the cells themselves provide an environment and the repetitive sequences may be subjected to selection pressure within the cell. It can, therefore, be argued that these sequences may be maintained through non-phenotypic selection, and by a mechanism directed for the self preservation of these sequences. It is in this context that the term selfish DNA, was used for this repetitive DNA.