Why are some genes active in only certain types of cells?

In multicellular organisms, every cell typically contains the same complete set of genes. The reason genes are active in only certain types of cells is due to the process of cell differentiation, which relies on the selective expression and repression of specific genes rather than the presence or absence of the genes themselves.12

This regulation is primarily controlled at the level of transcription, where transcription factors determine whether a gene is copied into RNA. These factors interact with regulatory sequences in the DNA to either activate or inhibit gene expression. Additionally, the physical structure of chromatin—the complex of DNA and proteins—plays a critical role; genes are generally only accessible for transcription when chromatin is in an "open" configuration.132

Epigenetic modifications, such as DNA methylation, further ensure that these patterns of gene activity remain stable and persistent as cells divide and mature into specialized types.14