[This post is in English because I felt like doing so ...]
While working with some recently sequenced genomic data, and figuring that centromere data is missing in most sequencing efforts, I learned about euchromatin today.
Euchromatin is the lightly packed DNA that is available for transcription. The other parts of our genome (called heterochromatin), which consists of the centromere and telomeres, is densely packed and not easily accessibly for proteins. The centromere sequence is made up of large arrays of DNA repeats, and is hypothesized not to be transcribed and thus functionless. Specific histones ("CEN" proteins) bind to the centromeres and cause a specific folding of the centromeres.
Because centromeres consist of large arrays of DNA repeats, it is not easy to sequence these with current methodologies, so the centromeres result in huge gaps (sometimes over 3 megabases) in sequencing efforts.