Gene silencing  refers to a mechanism by which cells pack up large sections of chromosomal DNA. It is generally wont to describe the “switching off” of a gene by a mechanism aside from genetic modification. That is, a gene which might be expressed (turned on) under normal circumstances is transitioned by machinery within the cell. Gene silencing is completed by incorporating the DNA to be silenced into a sort of DNA called heterochromatin that's already silent.

Gene silencing may be a general term describing epigenetic processes of gene regulation. This process is important for the differentiation of many different types of cells. Genes are regulated at either the transcriptional or post-transcriptional level. Transcriptional gene silencing is that the results of histone modifications, creating an environment of heterochromatin around a gene that creates it inaccessible to transcriptional machinery (RNA polymerase, transcription factors, etc.).