Ring chromosome 18 is a rare chromosome abnormality in which the ends (arms) of chromosome 18 join together to form a ring shape. When a ring chromosome forms, genetic material can be lost from either arm or both arms, causing various signs and symptoms. While most people with Ring chromosome 18 have the ring chromosome in all of their body cells, some people also have some body cells with normal chromosomes (this is called mosaicism). People with Ring chromosome 18 mosaicism may have milder symptoms. Depending on the amount of genetic material lost and which genes are involved, signs and symptoms that may be present in a person with Ring chromosome 18 include ear and hearing abnormalities, abnormally-developed feet or hands, genital abnormalities in males, an increased risk to develop juvenile rheumatoid arthritis, holoprosencephaly, developmental delays, learning difficulties, short stature, and/or various birth defects. Ring chromosome 18 usually occurs sporadically (by chance) during the formation of egg or sperm cells or shortly after the egg and sperm join together. Occasionally, it is inherited from a parent (typically the mother). A chromosome test of the parents can help determine whether it was inherited and whether future children have an increased chance to have a chromosome abnormality.