Ring chromosome formation can occur without deletion, through the abnormal pairing of the palindromic DNA base sequences thought to make up the telomeres. The normal occurrence of sister chromatid exchanges within a ring constantly produces further chromosomal anomalies that are less likely to survive when the chromosome involved is large and/or its aneuploidy incompatible with life. These abnormal products (abbreviated pro) were only found in the present case after two or more cell cycles in lymphocyte cultures. Their elimination in vivo implies a very high cellular death rate and an enormous waste of metabolism that should have the same phenotypic effects no matter what chromosome is involved. These phenotypic anomalies form a ring syndrome that can be clinically recognized and consists of severe growth failure, mental retardation and a pleasant personality. The syndrome is usually masked by the more severe abnormalities produced by the deletions present in most cases of ring chromosomes.