Regulation of gene expression occurs at multiple points. Chromatin structure is based on successive levels of dna packing. Eukaryotic dna is precisely combines with a large amount of protein. Eukaryotic chromosomes contain an enormous amount of dna relative to their condense length. Interphase chromatin is usually much less condensed than that of mitotic chromosomes: highly condensed areas are called heterochromatin, usually not expressed, less compacted areas are called euchromatin, are expressed. Regulation of chromatin structure: chemical modifications to histones and dna of chromatin influence both chromatin structure and gene expression, the histone code hypothesis proposes that specific combinations of modifications help determine chromatin configuration and influence transcription. In some species, dna methylation causes long-term inactivation of genes in cellular differentiation. In the genomic imprinting, methylation turns off either the maternal or paternal alleles of certain genes at the start of development.