Macrosatellite epigenetics: the two faces of DXZ4 and D4Z4

Almost half of the human genome consists of repetitive DNA. Understanding what role these elements have in setting up chromatin states that underlie gene and chromosome function in complex genomes is paramount. The function of some types of repetitive DNA is obvious by virtue of their location, such as the alphoid arrays that define active centromeres. However, there are many other types of repetitive DNA whose evolutionary origins and current roles in genome biology remain unknown. One type of repetitive DNA that falls into this class is the macrosatellites. The relevance of these sequences to disease is clearly demonstrated by the 4q macrosatellite (D4Z4), whereupon contraction in the size of the array is associated with the onset of facioscapulohumeral muscular dystrophy. Here, I describe recent findings relating to the chromatin organization of D4Z4 and that of the X-linked macrosatellite DXZ4, highlighting the fact that these enigmatic sequences share more than a similar name.