Small supernumerary marker chromosomes (sSMC) are present in ~2.7 million people worldwide [1]. They are defined as structurally abnormal chromosomes that cannot be identified or characterized unambiguously by conventional banding cytogenetics alone; they are generally equal in size or smaller than a chromosome 20 in the same metaphase spread. Additionally, sSMC can be present in an otherwise normal karyotype, in a numerically abnormal karyotype (e.g., Turner’s or Down’s syndromes) or in a structurally abnormal but balanced karyotype with or without ring chromosome formation. If detected in banding cytogenetics, they are a major problem as they are too small to be characterized for their chromosomal origin or content by traditional banding techniques. Molecular cytogenetic techniques are necessary for their characterization [2]. Cases with a de novo sSMC, particularly those that are prenatally ascertained, are not easy to correlate with a clinical outcome [3]. It has been established that substantial parts of sSMC lead to four specific syndromes, i.e., Pallister-Killian [= i(12p)], isochromosome 18p [i(18p)], cat-eye [i(22p~q)] and derivative chromosome 22 [der(22) t(11;22)] syndromes [2]. In general, the risk for an abnormal phenotype in prenatally ascertained de novo cases with sSMC is considered to be ~13% [4]. This has been refined to 7% (for sSMC from chromosomes 13, 14, 21 or 22) and 28% (for all non-acrocentric autosomes) [5] and has recently been suggested to be 26-30% [1,6]. Also, generally speaking, sSMC transmitted by normal sSMC carriers to their progeny are not correlated with clinical problems [7], although exceptions have been described [8].