AP-1, or Activator Protein 1, is a group of proteins that play a crucial role in regulating gene expression. It is composed of various proteins, including members of the Fos and Jun families, which form dimers to bind to specific DNA sequences. AP-1 is involved in numerous cellular processes, such as cell growth, differentiation, and response to stress. AP-1 is activated by various signals, including growth factors and stress stimuli, which can lead to its involvement in inflammation and cancer. By regulating the expression of target genes, AP-1 influences important biological functions, making it a significant focus of research in molecular biology and medicine.