[A small supernumerary metacentric chromosome: interprétation test (author's transl)].

The report relates to a new case of small supernumerary metacentric chromosome. The phenotypical variability of previously published cases suggests that this chromosomal anomaly may arise from multiple origins. Our study shows the difficulties of a cytogenetic interpretation and the limits of the semeiologic arguments. It is, however, possible to hypothesize an 18 partial trisomy.