Transmission of a ring chromosome 18 from a mother with 46,XX/47,XX, + r(18) mosaicism to her daughter, resulting in a 46,XX,r(18) karyotype.

A 6 month old patient is reported with a ring chromosome 18 confirmed by cytogenetic studies and in situ hybridisation. Her clinical features were similar to previous cases of ring chromosome 18 syndrome. The ring chromosome was inherited from the phenotypically and mentally normal mother with a mos 46,XX/47,XX, + r(18) karyotype.