Ring Chromosome 1

Human chromosome 1 is the largest chromosome and comprises ~249 million base pairs (Mb). Constitutional ring chromosome 1 (RC1) is an ultra-rare disorder with less than ten cases described in the scientific literature since the 1960s, when conventional karyotyping was incorporated into clinical practice. This chapter summarizes the clinical and cytogenetic data of eight published cases with RC1. The major phenotype includes severe growth retardation, congenital microcephaly, and global developmental delay. Dysmorphic features are generally non-specific, and congenital anomalies are highly variable. There is a potential predisposition to malignancy. Comprehensive cytogenomic analysis should be performed to define the dynamic mosaicism and genomic structure for RC1. Genetic counseling and symptomrelated medical management need to be provided to patients and families with RC1.