As they age, neurological deficits begin to manifest with seizures, dystonias, and severe developmental delay.

Other mutant alleles of the FH gene, located on human chromosome 1 at position 1q42.1, cause multiple cutaneous and uterine leiomyomata, hereditary leiomyomatosis and renal cell cancer.

[6] The condition is an autosomal recessive disorder,[7] and it is therefore usually necessary for an affected individual to receive the mutant allele from both parents.

[11][12][13][14] Nicknamed "Polygamist's Down's", the syndrome has been blamed on cousin marriage, but in a larger sense is related to the reproductive isolation of a community among whom 85% are blood relatives of John Y. Barlow and/or Joseph Smith Jessop (the cofounders of the Short Creek Community).

[12] Since the initial cluster from FLDS communities was reported, it is now estimated that there are 100 documented cases worldwide.