Accumulation: The process of acquiring more or the build-up of a substance.

Allele: An alternative form of a gene that differs in DNA sequence and structure.

Anemia: A condition in which the body does not have enough healthy red blood cells.

Arteries: Red blood vessels that carry blood away from the heart.

Ataxia: Failure of muscular coordination; irregularity of muscle action.

Autosomal recessive inheritance: Affected individuals inherit two non-working copies of a gene, one from each parent. Parents are carriers, meaning they have one non-working copy of the gene and one working copy. The parents have a 1 in 4 chance of having an affected child with each pregnancy.

Bile Acids: Fatty substances made in the liver from cholesterol, and important for digestion of fat and fat soluble vitamins.

Cataracts: A clouding that develops in the crystalline lens of the eye

Cell: The basic unit of all organisms. Cells can be of several different types depending on function [e.g. macrophages (a type of white blood cell), neuron (nerve cell), hepatocyte (liver cell) erythrocyte (red blood cell)].

Chenodeoxycholic acid: A bile acid.

Cholestanol: A cholesterol derivative.

Cholesterol: An essential component of cells, involved in the production of bile acids, steroid hormones and fat soluble vitamins.

Cleft palate: A condition in which the two plates of the skull that form the hard palate (roof of the mouth) are not completely joined

Dementia: A loss of brain function that affects memory, thinking, language, judgment, and behavior.

Dystonia: A neurological movement disorder, in which sustained muscle contractions cause twisting and repetitive movements or abnormal postures.

DNA (deoxyribonucleic acid): The molecule that encodes genetic information.

Enzyme: A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds (helps it happen more quickly and efficiently).

Gene: The fundamental physical and functional unit of heredity. A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product (i.e., a protein or RNA molecule). See gene expression.

Gene expression: The process by which a gene's coded information is converted into the molecules present and operating in the cell. Expressed genes include those that are transcribed (copied) into mRNA and then translated into protein and those that are transcribed into RNA but not translated into protein (e.g. transfer and ribosomal RNAs)

Genotype: The genetic makeup of an individual; usually refers to the DNA structure of a particular gene being analyzed.

Gene Product: The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease-causing alleles.

Genetic Code: The sequence of nucleotides, coded in triplets (codons) along the mRNA that determines that sequence of amino acids in protein synthesis. The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

Hypotonia: Decrease in muscle tone.

Ichthyosis: Dry, thickened, scaly or flaky skin.

Jaundice: Yellowish discoloration of the skin.

Lipid: A broad group of naturally occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

Microcephaly: A neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex.

Polydactyly: Extra fingers or toes

Protein: A large molecule composed of one or more chains of amino acids in a specific order; the order is determined by the base sequence of nucleotides in the gene coding for the protein. Proteins are required for the structure, function, and regulation of the body’s cells, tissues, and organs, and each protein has unique functions. Examples are hormones, enzymes, and antibodies.

Ptosis: Also called "drooping eyelid." It is caused by weakness of the muscle responsible for raising the eyelid, damage to the nerves that control those muscles, or looseness of the skin of the upper eyelids.

Sterols: An important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol. Vitamin D is made from sterols. Cholesterol is vital to cellular function, and a precursor to steroid hormones.

Spasticity: Impaired fluid movement of the arms or legs caused by abnormal nerve function.

Tendon Xanthomas: A deposition of yellowish cholesterol-rich material in tendons or other body parts in various disease states.