According to the current model of nuclear organization, based on several decades of morphological studies, there are two main structural components of the cell nucleus: a chromatin and an interchromatin compartment (reviewed by Rouquette et al. (2010)). Such an arrangement was originally proposed by Monneron and Bernhard (1969) who distinguished the two components simply on the basis of the electron density (e.g. the dense chromatin and the lucent interchromatin space). Later studies, using more sophisticated methodology, including fluorescent in situ hybridization (FISH) and three-dimensional (3D) analysis by superresolution microscopy, fully supported the two-compartment model, and demonstrated its physiological relevance (Markaki et al., 2010, 2012; Schermelleh et al., 2008).