The rate of sister chromatid exchange in normal human bone marrow cells

Summary A ring chromosome 22 is described in a 6-year-old mentally retarded boy, who presented a dysmorphic syndrome. The ring chromosome 22 was inherited from the mother, in whom a 46,XX/46,XX,r(22)/45,XY,-15,-22,+t(15;22)(p11;q11) mosaic karyotype was found, indicating a high degree of instability of the chromosome(s) 22 in this woman.