The genome of Drosophila melanogaster, a fruit fly, was sequenced in 2000. However, this sequence consisted mainly of euchromatin. The heterochromatin was not sequenced until 2007. Most completed genome sequences do not include heterochromatin. Why is heterochromatin usually not sequenced in genomic projects

Heterochromatin appears as dark staining, small, and irregular particles dispersed all through the nucleus or are gathered close to the nuclear envelope. Heterochromatin is mostly found in the cells, which are not or less active. The most completed sequences of genome do not comprise heterochromatin as heterochromatin constitutes various short repeated sequences that make it tough to align the sequences of heterochromatin into big contigs.