A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryote cell. The term is also used of the complete set of chromosomes in a species, or an individual organism. It describes the number of chromosomes, and what they look like under a light microscope. Attention is paid to their length, the position of the centromeres, any differences between the sex chromosomes, and any other physical characteristics. The preparation and study of karyotypes is part of cytogenetics (cytology and genetics).