Speckles are subnuclear structures that are enriched in pre-messenger RNA 
splicing factors and are located in the interchromatin regions of the 
nucleoplasm of mammalian cells. At the fluorescence-microscope level they appear 
as irregular, punctate structures, which vary in size and shape, and when 
examined by electron microscopy they are seen as clusters of interchromatin 
granules. Speckles are dynamic structures, and both their protein and 
RNA-protein components can cycle continuously between speckles and other nuclear 
locations, including active transcription sites. Studies on the composition, 
structure and behaviour of speckles have provided a model for understanding the 
functional compartmentalization of the nucleus and the organization of the 
gene-expression machinery.