The descriptive term “heterochromatin” was introduced by the botanist Emil Heitz in 1928 for chromosomal regions that remain condensed during interphase and stain intensively in contrast to euchromatin. Later a distinction was made between facultative heterochromatin (a transient state) and constitutive heterochromatin (Brown, 1966). The constitutive heterochromatin is a permanent entity of a chromosome and according to text book opinion considered “to be DNA that is never transcribed in any cell” (Alberts et al., 1983). Even more, the possibility was suggested “that constitutive heterochromatin per se has no function in either development or evolution” (John, 1988). It is a constituent of both plant and animal genomes. For mammals, the gonosomal c-heterochromatin of the European field vole Microtus agrestis has been considered typical for constitutive heterochromatin in general: it is C-band positive, late replicating, and transcriptionally inactive in various tissues (Sieger et al., 1970).