A further microchromosome with centromeric association.

A 46,XY/47,XY,+mar karyotype was found in a 5-year-old boy with mental retardation and minor dysmorphisms. The marker was present in 68% of lymphocyte metaphases, was about the size of Yp, appeared pale with G- and C-bands and had a single pair of centromeric dots; it was never seen in two or more copies and appeared larger and clearly annular in a few cells. This microchromosome was associated (interchromosomal distance equal to or smaller than 18p) with any other chromosome(s) in 295/344 cells. Among a total of 457 associations, 240 (52%) were centromeric. Such a proportion as well as the mostly random distribution of centromeric associations was similar to 3 previous instances. The paucity of markers with centromeric association may reflect a relative unawareness on the subject but more probable indicates that such a phenomenon is confined to nonsatellited, monocentric and annular microchromosomes.