[Trisomy 9p. Apropos of 2 cases].

Two cases of trisomy 9p are presented. The different cytogenetic mechanism given in these cases shows us that with independence of it, it exists a triplication of the half distal short arm of chromosome 9 which gives specificity to these phenotypic features. It might be a possible meiotic origin of the chromosomic rearrangement on both translocations, the formation of satellited chromosomes and isochromosomes of the short arm on chromosome 9 and the consequently wrong segregation of each one of them. The non-existence of chromosomic material with activity in the transcription, confers to both situations the category of a pure trisomy 9p. The prenatal diagnosis through amniocentesis might be useful, even in the so called "de novo" cases, in order to avoid the repetition of this kind of structural aberration or some others that could appear due to fragility of certain chromosomic regions.