Macroorchidism is a disorder found in males where a subject has abnormally large testes. The condition is commonly inherited in connection with fragile X syndrome, which is also the second most common genetic cause of mental disabilities. The opposite side of the spectrum is called microorchidism, which is the condition of abnormally small testes.

Other possible causes of macroorchidism are long-standing primary hypothyroidism, adrenal remnants in congenital adrenal hyperplasia, follicle stimulating hormone (FSH)-secreting pituitary macroadenomas, local tumors, lymphomas, and aromatase deficiency.

A 2013 review stated that life expectancy for FXS was 12 years lower than the general population and that the causes of death were similar to those found for the general population.

Fragile X syndrome is a genetic disorder which occurs as a result of a mutation of the "fragile X mental retardation 1" ("FMR1") gene on the X chromosome, most commonly an increase in the number of CGG trinucleotide repeats in the 5' untranslated region of "FMR1". Mutation at that site is found in 1 out of about every 2000 males and 1 out of about every 259 females. Incidence of the disorder itself is about 1 in every 3600 males and 1 in 4000–6000 females. Although this accounts for over 98% of cases, FXS can also occur as a result of point mutations affecting "FMR1".

In unaffected individuals, the "FMR1" gene contains 5–44 repeats of the sequence CGG, most commonly 29 or 30 repeats. Between 45-54 repeats is considered a "grey zone", with a premutation allele generally considered to be between 55 and 200 repeats in length. Individuals with fragile X syndrome have a full mutation of the "FMR1" allele, with over 200 CGG repeats. In these individuals with a repeat expansion greater than 200, there is methylation of the CGG repeat expansion and "FMR1" promoter, leading to the silencing of the "FMR1" gene and a lack of its product.

This methylation of "FMR1" in chromosome band Xq27.3 is believed to result in constriction of the X chromosome which appears 'fragile' under the microscope at that point, a phenomenon that gave the syndrome its name. One study found that FMR1 silencing is mediated by the FMR1 mRNA. The FMR1 mRNA contains the transcribed CGG-repeat tract as part of the 5' untranslated region, which hybridizes to the complementary CGG-repeat portion of the FMR1 gene to form an RNA·DNA duplex.

Trinucleotide repeat disorders (also known as trinucleotide repeat expansion disorders, triplet repeat expansion disorders or codon reiteration disorders) are a set of genetic disorders caused by trinucleotide repeat expansion, a kind of mutation where repeats in certain genes or introns exceed the normal, stable threshold, which differs per gene. The mutation is a subset of unstable microsatellite repeats that occur throughout all genomic sequences. If the repeat is present in a healthy gene, a dynamic mutation may increase the repeat count and result in a defective gene. If the repeat is present in an intron it can cause toxic effects by forming spherical clusters called RNA foci in cell nuclei.

Trinucleotide repeats are sometimes classified as insertion mutations and sometimes as a separate class of mutations.

Since the early 1990s, a new class of molecular disease has been characterized based upon the presence of unstable and abnormal expansions of DNA-triplets (trinucleotides). The first triplet disease to be identified was fragile X syndrome, which has since been mapped to the long arm of the X chromosome. At this point, there are from 230 to 4000 CGG repeats in the gene that causes fragile X syndrome in these patients, as compared with 60 to 230 repeats in carriers and 5 to 54 repeats in unaffected individuals. The chromosomal instability resulting from this trinucleotide expansion presents clinically as intellectual disability, distinctive facial features, and macroorchidism in males. The second, related DNA-triplet repeat disease, fragile X-E syndrome, was also identified on the X chromosome, but was found to be the result of an expanded CGG repeat. Identifying trinucleotide repeats as the basis of disease has brought clarity to our understanding of a complex set of inherited neurological diseases.

As more repeat expansion diseases have been discovered, several categories have been established to group them based upon similar characteristics. Category I includes Huntington's disease (HD) and the spinocerebellar ataxias that are caused by a CAG repeat expansion in protein-coding portions of specific genes. Category II expansions tend to be more phenotypically diverse with heterogeneous expansions that are generally small in magnitude, but also found in the exons of genes. Category III includes fragile X syndrome, myotonic dystrophy, two of the spinocerebellar ataxias, juvenile myoclonic epilepsy, and Friedreich's ataxia. These diseases are characterized by typically much larger repeat expansions than the first two groups, and the repeats are located outside of the protein-coding regions of the genes.