The detection of an SMC is nearly always unexpected by the clinician, and more or less an accidental result in cytogenetics. To facilitate the characterization of SMC according to the novel molecular-cytogenetic possibilities, we here propose a strategy for SMC-characterization (see Fig. 1).

Supernumerary marker chromosomes are initially detected in a GTG-banding analysis and can be character­ized for the presence of an acrocentric-chromosome-de­rived short arm by NOR-staining. According to the NOR-staining result (NOR-negative or NOR-positive) the origin of the SMC can be determined by application of all hu­man centromeres in one experiment (i.e., cenM-FISH [8]) or by the acrocenM-FISH probe set [9], respectively. These approaches have been successfully used in about 100 cases with SMC [8,13-14,18-22; unpublished data].

In general, neocentromeric SMC are not stained by any of the two aforementioned approaches; in this case other approaches for their characterization, such as M-FISH [17], microdissection of the SMC [e.g., 23] or micro-CGH [23], have to be performed. About 60 cases have been reported up to present in the literature with neocentromeres (for review see [24]).

After determination of the origin of a small SMC the most important question to address is, is there euchromatic material of the corresponding chromosome on it, or is there not? This question for partial trisomy can be studied by hybridizing whole chromosome painting (WCP) or chromosome arm specific probes [e.g., 22]. Another possi­bility is the use of probes that are able to subclassify chro­mosomal regions [e.g., 23] or MCB [12,14,17]. At pres­ent, we are working on a probe set specific for the peri-centric region of all human chromosomes, the subcenM-FISH technique [10]. In the strategy proposed in Fig. 1, MCB is suggested for the characterization of the euchro­matic content of SMC larger than 17p and subcenM-FISH for the smaller ones (see Fig. 1).

Various mechanism of formation of small SMC have been proposed: trisomy rescue, monosomy rescue, post fertilization errors and gamete complementation. Recently, another mechanism was proposed, which specu­lates that SMC are derived from a transfection of the chro­mosome into the zygote derived from superfluous haploid pronucleus that is usually degraded by deoxyribonucleases or other means [25]. Meta analysis of SMC associated with UPD revealed functional trisomy rescue as the most likely mechanism in marker chromosome formation [5]. The underlying mechanism is always two events: either two meiotic, or one meiotic and one mitotic, which lead to the inheritance of both homologous chromosomes from only one parent. Uniparental disomy has been reported for the majority of human chromosomes [4,26]. Therefore, in the case of identification and characterization of a small SMC, an exclusion of the UPD of the two normal sister chromosomes by molecular genetic methods is reasonable [13,18,22].

In summary, as none of the available approaches can, for technical reasons, ever be fully informative by itself, the cytogenetist always has to combine the available meth­ods, depending on the problem to be addressed. GTG-banding is still the gold standard, which as well the start­ing point for any molecular cytogenetic for the character­ization of small SMC; acrocenM-FISH characterizes its origin, MCB or subcenM-FISH gives information on addi­tional euchromatin and molecular genetics uncovers even­tually present UPDs. The strategy proposed here (Fig. 1) might be changed again, depending on further develop­ments of available methods, or an increase in our knowl­edge of SMC.