C-to-U editing is a biological process where a specific type of RNA, known as messenger RNA (mRNA), undergoes a change at the molecular level. In this process, a cytidine (C) nucleotide is converted into a uridine (U) nucleotide. This editing can affect how proteins are produced from the mRNA, potentially altering their function and activity. This editing is primarily facilitated by enzymes called ADARs (adenosine deaminases acting on RNA) and is crucial for various cellular functions. C-to-U editing plays a significant role in the regulation of gene expression and can influence processes such as neurotransmission and immune responses.