Amino acid – The chemical unit that is the building block of proteins. Some examples are cysteine, valine, phenylalanine, guanine, aspartate

Ataxia – Uncoordinated movements

Athetoic movements – Writhing movements

Axon – Projection of a neuron (nerve cell) that carries nerve impulses from one cell body to another


Base – The chemical unit that is the building block of DNA (deoxyribonucleic acid); the four bases in DNA are adenine, thymine, guanine, cytosine, represented by the letters A, T, G, and C, that form the codons (three-letter words) of the genetic code

Benign – Not disease-causing


Carrier – A carrier has a disease-causing variant (mutation) on one chromosome of a pair and normal copy on the other chromosome of the pair. The normal copy compensates for the disease-causing variant and the person is not affected. Female carriers of a variant that causes PMD in males on one of their two X chromosomes are usually not affected. Males are not carriers because they only have one X chromosome.

Cell – A basic unit or building block that is able to perform all of the functions of life

Codon – A three-letter (base) “word” of the genetic code. A codon codes for one amino acid; for example, AGC codes for serine, which can be abbreviated Ser or S.

Connatal – Congenital; at birth


DNA – Deoxyribonucleic acid that contains the code for making proteins; forms a double-stranded helix

Dystonic posturing – A movement disorder with muscle contractions resulting in abnormal fixed postures.

Dysarthria – Slow speech


Exon – The exons are the part of a gene that are left in the RNA after splicing occurs, and they code for protein.



Gastrostomy – Surgery for the placement of a feeding tube


Hypotonia – Muscle weakness


Intron – The part of a gene that is spliced out of the RNA




Leukodystrophy – A disease of the white matter of the central nervous system (brain and spinal cord)


Magnetic resonance imaging (MRI) – A medical imaging technique that produces images of internal organs, such as the brain

Mutation – A base-change in the DNA; sometimes thought of as a disease-causing (pathogenic) change

Myelin – Fatty substance covering and insulating the axons of neurons so that nerve impulses travel properly from neuron (nerve cell) to neuron

Myelin sheath – Myelin that is wrapped around the axons of neurons (nerve cells) forming a covering 


Neuron – Nerve cell

Nystagmus – Involuntary eye movements


Oligodendrocyte – Type of brain cell that produces myelin in its cell membrane and sends out projections of its cell membrane to wrap around axons of neurons (nerve cells) forming an insulating sheath


Pathogenic – Disease-causing

Protein – A chemical made of a chain of amino acids that provides a structural function or assists chemical reactions to occur in living things



RNA – Ribonucleic acid; transcription of the DNA code into RNA is an intermediate step between DNA and formation of protein


Spasticity – Tightness or stiffness of muscles

Splicing – Process by which introns are removed from RNA leaving the exons, which code for protein

Stridor – Characteristic sound of breathing when airflow is disrupted in the airway


Titubation – Head-bobbing



Variant – A base change; can be pathogenic (disease-causing) or benign (not disease-causing)


X-inactivation – The inactivation of genes on one copy of a pair of X chromosomes. Which of the two Xs is inactivated is usually random from cell to cell.