CpG refers to a specific sequence of DNA where a cytosine (C) nucleotide is followed by a guanine (G) nucleotide, connected by a phosphate bond. This sequence is important in genetics because it often occurs in regions of DNA that regulate gene expression. The "p" in CpG stands for the phosphate group that links the two nucleotides. In the human genome, CpG sites are often found in promoter regions, which control the activity of genes. Methylation of these sites can influence gene expression, playing a crucial role in processes like development, cell differentiation, and disease mechanisms, including cancer.