Small supernumerary marker chromosome

A small supernumerary marker chromosome (sSMC) is an abnormal extra chromosome. It contains copies of parts of one or more normal chromosomes and like normal chromosomes is located in the cell's nucleus, is replicated and distributed into each daughter cell during cell division, and typically has genes which may be expressed. However, it may also be active in causing birth defects and neoplasms (e.g. tumors and cancers). The sSMC's small size makes it virtually undetectable using classical cytogenetic methods: the far larger DNA and gene content of the cell's normal chromosomes obscures those of the sSMC. Newer molecular techniques such as fluorescence in situ hybridization, next generation sequencing, comparative genomic hybridization, and highly specialized cytogenetic G banding analyses are required to study it. Using these methods, the DNA sequences and genes in sSMCs are identified and help define as well as explain any effect(s) it may have on individuals. Human cells typically have 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Each member of the paired autosomal chromosomes is identified as chromosome 1 up to 22; the pair of sex chromosomes are identified as the X and Y chromosomes with women's cells bearing two X chromosomes and men's cells bearing one X and one (male sex-determining) Y chromosome. sSMC are, by definition, smaller in size than one of the smaller human chromosomes, chromosome 20. They originate as copies of relatively small parts of one or more of the 46 chromosomes. Not all chromosomes are equally represented in sSMCs: ~65% of all sSMCs are copies of parts of chromosome 15 while only 7% are copies of parts of one of the five acrocentric chromosomes viz., chromosomes 13, 14, 15, 21, and 22. G banding analyses of sSMCs are commonly used to identify the chromosomes from which they were derived, the arms of these chromosomes ("p" for short arm, "q" for long arm) they contain, and the parts of these chromosomes' arms they have as defined by their G band contents.