Organization of constitutive heterochromatin. (A) Constitutive heterochromatin is found at pericentromeric, telomeric, and ribosomal regions, as well as at different loci along the chromosome (B) Centromeres and pericentromeres consist of predominantly repetitive DNA sequences, including simple repeats, DNA transposons, LTR-endogenous retroviral elements, non-LTR autonomous retrotransposons including long interspersed elements and short interspersed elements. The approximate length of the different repetitive elements is indicated. In mice, centromeres consist mainly of minor satellites and pericentromeres of major satellites. In human beings, centromeres consist mainly of alpha satellites and pericentromeres of chromosome specific satellite repeats, including satellites I, II and III. (C) Chromocenters in differentiated cells are smaller, more numerous and more condensed than chromocenters in undifferentiated pluripotent cells, which are probably more dynamic. CT, centromere; DR, direct repeat; IR, inverted repeats; LINE, long interspersed element; MEF, mouse embryonic fibroblasts; mESC, mouse embryonic stem cells; PCT, pericentromere; SINE, short interspersed element; TSDR, target site direct repeat.