Regulation of gene expression involves a number of different levels of organization in the cell nucleus. The main agents of transcriptional control are the cis-acting sequences in the immediate vicinity of a gene, which combine to form the functional unit or domain. Contacts between these sequences through the formation of chromatin loops forms the most basic level of organization. The activity of functional domains is also influenced by higher order chromatin structures that impede or permit access of factors to the genes. Epigenetic modifications can maintain and propagate these active or repressive chromatin structures across large genomic regions or even entire chromosomes. There is also evidence that transcription is organized into structures called 'factories' and that this can lead to inter-chromosomal contacts between genes that have the potential to influence their regulation.