Carnosinemia is a rare autosomal recessive metabolic disorder caused by a deficiency of carnosinase, a dipeptidase (a type of enzyme that splits dipeptides into their two amino acid constituents).Carnosine is a dipeptide composed of beta-alanine and histidine, and is found in skeletal muscle and cells of the nervous system. This disorder results in an excess of carnosine in the urine, cerebrospinal fluid, blood, and nervous tissue. Neurological disorders associated with a deficiency of carnosinase, and the resulting carnosinemia ("carnosine in the blood") are common. Symptoms and signs
A variety of neurological symptoms have been associated with carnosinemia. They include: hypotonia, developmental delay, intellectual disability, degeneration of axons, sensory neuropathy, tremors, demyelinization, gray matter anomalies, myoclonic seizures, and loss of purkinje fibers. Genetics
The gene for carnosinase is located on chromosome 18, an autosome. The carnosine dipeptidase-1 gene (CNDP1) controls tissue and serum carnosinase. Mutations in CNDP1 are  responsible for carnosinase deficiency, resulting in carnosinemia.Carnosinemia is an autosomal recessive disorder, which means the defective gene is located on an autosome, and two copies of the defective gene - one from each parent - are required to inherit the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but usually do not experience any signs or symptoms of the disorder. Diagnosis
Types
Carnosinase in humans has two forms:1. Cellular, or tissue carnosinase: This form of the enzyme is found in every bodily tissue.
The upper lip may be "tented", exposing dental crowding, "tongue tie", or gum hypertrophy. Though the eyes do not protrude, abnormalities in the lower eyelid may give that impression. Surgery may be necessary if the eyes cannot close fully. In addition, the ears may be set low on the head or have a deformed pinna.Patients suffer from dwarfism, short lower arms, small feet, and small hands. Fingers and toes may also be abnormally short and laterally or medially bent. The thumb may be displaced and some patients, notably in Turkey, experience ectrodactyly. All patients often suffer from vertebral segmentation abnormalities. Those with the dominant variant have, at most, a single butterfly vertebra. Those with the recessive form, however, may suffer from hemivertebrae, vertebral fusion, and rib anomalies. Some cases resemble Jarcho-Levin syndrome or spondylocostal dysostosis.Genital defects characteristically seen in males include a micropenis with a normally developed scrotum and testes. Sometimes, testicles may be undescended, or the patient may suffer from hypospadias. Female genital defects may include a reduced size clitoris and underdeveloped labia minora. Infrequently, the labia majora may also be underdeveloped. Some research has shown that females may experience vaginal atresia or haematocolpos.The autosomal recessive form of the disorder tends to be much more severe. Examples of differences are summarized in the following table:

Associated conditions
Medical conditions include frequent ear infection, hearing loss, hypotonia, developmental problems, respiratory problems, eating difficulties, light sensitivity, and esophageal reflux.Data on fertility and the development of secondary sex characteristics is relatively sparse.