In humans, the presence of supernumerary chromosomes is an unusual phenomenon, which is often associated with developmental abnormalities and malformations. In contrast to most animal and plant species, the extensive knowledge of the human genome and the ample set of molecular and cytogenetic tools available have permitted to ascertain not only that most human supernumerary chromosomes (HSCs) derive from the A chromosome set, but also the specific A chromosome from which most of them arose. These extra chromosomes are classified into six types on the basis of morphology and size. There are both heterochromatic and euchromatic HSCs, the latter being more detrimental. Most are mitotically stable, except some producing individual mosaicism. No information is available on the HSC transmission rate since extensive familial studies are not usually performed generally because of death of the relatives or lack of cooperation. The main B chromosome property failing in HSCs seems to be their population spread as polymorphisms, since most HSCs seem to correspond to extra A chromosomes or centric fragments spontaneously arisen in the analysed individual or one of his/her parents. However, we cannot rule out at this moment, that more intensive studies on population distribution and frequency of those HSCs most closely resembling B chromosomes (i.e. those heterochromatic and thus less detrimental) would reveal possible HSCs polymorphisms. Although HSCs cannot be considered B chromosomes, some of them might be a source for future B chromosomes. The best candidates would be heterochromatic HSCs, which might manage to drive in either sex. To ascertain this possibility, research on inheritance and population studies would be very helpful in combination with the powerful cytogenetic and molecular tools available for our species. Copyright © 2004 S. Karger AG, Basel.