The Pallister–Killian syndrome (PKS), also termed tetrasomy 12p mosaicism or the Pallister mosaic aneuploidy syndrome, is an extremely rare and severe genetic disorder. PKS is due to the presence of an extra and abnormal chromosome termed a small supernumerary marker chromosome (sSMC). sSMCs contain copies of genetic material from parts of virtually any other chromosome and, depending on the genetic material they carry, can cause various genetic disorders and neoplasms. The sSMC in PKS consists of multiple copies of the short (i.e. “p”) arm of chromosome 12.^[1] Consequently, the multiple copies of the genetic material in the sSMC plus the two copies of this genetic material in the two normal chromosome 12’s are overexpressed and thereby cause the syndrome.^[2] Due to a form of genetic mosaicism, however, individuals with PKS differ in the tissue distributions of their sSMC and therefore show different syndrome-related birth defects and disease severities. For example, individuals with the sSMC in their heart tissue are likely to have cardiac structural abnormalities while those without this sSMC localization have a structurally normal heart.^[3]

PKS was first described by Philip Pallister in 1977 and further researched by Maria Teschler-Nicola and Wolfgang Killian in 1981.^[4]

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