Many students in special education programs led by special education degree holders are labeled as having learning difficulties or learning disabilities. A learning difficulty or learning disability is commonly referred to as an LD, and this term means that the student has a problem learning in one or more areas caused by brain dysfunction, other disabilities, a brain injury, or even developmental aphasia (lack of language abilities). There is a spectrum of LDs from mild to moderate to severe to profound. LDs are difficult to define, however. Even under federal laws related to special education, the definition of an LD focuses on what it is not rather than what it is. For the most part, the working definition of an LD is a divergence between a student's abilities, achievements or IQ, and his or her actual achievement. There is no one standard or test for this discrepancy, making the definition of what an LD is vary widely from person to person. Sometimes, the definition of an LD can include mental retardation, which means having a significantly under-functioning intellect and having deficiencies in adaptive behavior, and/or suffering from disorders like Down syndrome, a chromosomal defect that profoundly affects both physical and intellectual development and creates other medical problems. Students with LDs often benefit from specific teaching strategies geared towards their unique problem.