[3] Schwartz–Jampel syndrome is caused by mutations in the HSPG2 gene, which makes the protein perlecan, which is found in muscle and cartilage.

[4] In Schwartz–Jampel syndrome, it is suspected that abnormal perlecan function leads to a deficiency in acetylcholinesterase, an enzyme involved in breaking down the neurotransmitter acetylcholine, which incites muscle contraction.

[5] Some signs and symptoms that are frequently exhibited in patients with SJS include epiphyseal abnormalities, metaphyseal abnormalities, arthrogryposis multiplex congenita, bowing of long bones, a protruding lower lip, full cheeks, a disturbed gait, genu valgum, hip dysplasia, hypertonia, intellectual disability, joint stiffness, low-set posteriorly-rotated ears, metatarsus valgus, micromelia, myotonia, narrow mouth, flat foot, pursed lips, short stature, skeletal dysplasia, trismus, and visual impairment.

[6][7] In March 2013, there was media coverage of a British 7-year-old boy named Owen, with chronically tense muscles due to SJS, and his three-legged pet dog.

[12] In 2016, 9-year-old Owen was featured in a TLC documentary series, Two in a Million, in which he met and formed a friendship with Giovanni, a 7-year-old American boy with SJS.