RT Journal Article SR Electronic T1 Origins of accessory small ring marker chromosomes derived from chromosome 1 JF Journal of Medical Genetics JO J Med Genet FD BMJ Publishing Group Ltd SP 847 OP 853 DO 10.1136/jmg.36.11.847 VO 36 IS 11 A1 D F Callen A1 H Eyre A1 Y-Y Fang A1 X-Y Guan A1 A Veleba A1 N J Martin A1 J McGill A1 E A Haan YR 1999 UL http://jmg.bmj.com/content/36/11/847.abstract AB Three patients with accessory small ring chromosomes derived from chromosome 1 are presented together with additional clinical details and cytogenetic analyses of a previously reported patient. Cytogenetic analysis was undertaken by FISH using a reverse painting probe generated from one of the patients by microdissection of the r(1) chromosome and with a BAC923C6 which maps to 1p12. Results indicated that patients with r(1) chromosomes consisting of 1q12 heterochromatin and short arm pericentric euchromatin which extends to at least the BAC923C6 were associated with a normal or mild phenotype. Patients with abnormal phenotypes possessed two types of rings. One patient had evidence for contiguous pericentric short arm euchromatin which extended from the centromere to beyond the BAC923C6. Two patients showed molecular cytogenetic results which were compatible with non-contiguous chromosome 1 euchromatin. The diversity of origin of r(1)s will hamper attempts to define phenotype/genotype relationships.