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Two unrelated patients carrying imbalances involving the long arm of chromosome 6 are described. In the first trisomy 6q21 leads to qter had segregated from a maternal translocation t(6;16)(q15;q24). The clinical data of the proposita are compared with those of three other published cases. A partial 6q trisomy syndrome is postulated characterized by: growth deficiency of prenatal onset, psychomotor retardation, craniofacial abnormalities (microcephalia, hypertelorism, downward slanting palpebral fissures, flattened nasal bridge, long philtrum, hypoplastic perioral features, large jaw resulting in a round appearance of the face, receding chin, malformed ears) and dysmorphic extremities (contractures of limbs due to short flexor tendons, hypoplastic fingers, toes and nails). In the second case, monosomy 6q221 leads to qter resulted from a de novo rearrangement and was responsible for mental retardation and facial dysmorphism (reduced biparietal diameter, hypotelorism, absent eyebrows, prominent nose, ptosis, receding chin, dysmorphic ears). Studies of HLA and PGM3 segregation showed normal inheritance patterns and ruled out the location of these genes in bands 6q221 leads to qter.

A case of partial trisomy 9 is described in a mentally retarded and dysmorphic child, confirming that this chromosome unbalance results in a characteristic clinical entity. This trisomy arose through aberrant segregation of translocation chromosome during meiosis in the patient's mother, who is a balanced heterozygote for a complex translocation involving chromosomes 9, 21 and 22. The phenotypically normal sister of the proposition is also carrier of the same complex translocation.

Peripheral blood lymphocytes from eight Fanconi anemia (FA) patients, 14 FA heterozygotes, and nine normal subjects have been tested for their susceptibility to chromosomal breakage induction by diepoxybutane (DEB) and by two peroxides. In addition, the effect of five antioxidants was investigated in standard cultures and in cultures stressed either with DEB or with butylhydroperoxide (BHP) or with hydrogen peroxide (H2O2). DEB, BHP, and H2O2 dramatically increased the chromosomal breakage levels in homozygous and heterozygous FA cells. A partial correction of chromosomal instability was obtained by treating the patients' lymphocytes with antioxidants. A "protective" effect was also noted in the DEB or peroxide-stressed lymphocytes of patients and heterozygotes, grown in the presence of antioxidants.

Centric fission of chromosome No. 4 was found in the healthy mother of two children with trisomy 4p. The two telocentrics derived are stable and show no evidence of fusing again. The 2:3 ratio of unbalanced offsprings born to the proposita indicates that, contrary to the evidence emerging from studies in other species, the risk for the production of unbalanced gametes is high.