Introduction Intellectual disability (ID) is defined by the American Association of Intellectual Developmental Disabilities as having “significant limitations both in intellectual functioning and in adaptive behavior”.  Earlier it was believed that behavioral abnormalities, such as Intellectual Disability, were caused by the social or environmental environment in which the individual lived in. However, with the breakthrough of the genetic analysis, it has been shown that more and more of our behavior is influenced, if not controlled, by our genes. This advancement is important for our understanding of ID to expand.
ID can occur when there is a mutation on one or more of ...view middle of the document...
To find and analyze the gene that is mutated, different sequencing methods are used.  Exome sequencing is a relatively new technique of isolating and analyzing genes in the DNA.  It allows isolation of cleaved DNA fragments that then get sequenced for easier spotting of the genes that segregate from the genes of a healthy individual (not having a mutation). Exome sequencing is an appropriate gene-analysis method because it’s more time efficient than e.g. Sanger sequencing (in where one region of the DNA is read each time) in analyzing the coding regions of the genome (hereditary information of an organism). 
Isolating and understanding the genetic basis for the disability allows for new possibilities of drug treatment.
In the article Mutations in the Alpha 1,2-Mannosidase Gene, MAN1B1, Cause Autosomal-Recessive Intellectual Disability, Rafiq et al. present their findings of the gene that they investigated causes autosomal recessive intellectual disability [ARID].  In this study, the genetic setup of five families with high proportion of consanguinity (See Table 1) with individuals having ARID was analyzed. To find out which gene was the cause of the ID, a specific region of chromosome 9 was mapped for all affected members of the families by both Sanger sequencing and exome sequencing. It was found that the MAN1B1 gene was the one gene which all affected members had a mutation in, but none of the unaffected members; thus it was concluded that a mutation in this gene causes ARID. 
A mutation in the MAN1B1 gene creates a failure of encoding the protein endoplasmic reticulum mannosyl-oligosaccharide 1,2-alpha-mannosidase [ERManI], which functions as an enzyme (See Fig. 1).  Having two copies of the mutated MAN1B1 gene gives rise to malfunctioning ERManI enzymes. These enzymes are believed to play a role in the removal of misfolded glycoproteins, which travel through the endoplasmic-reticulum-associated degradation pathway [ERAD pathway]. Malfunctioning enzymes may result in incorrect processing of glycoproteins in the cell. 
Figure 1: showing a 3D model of the ERManI protein. The blue molecular structures indicate mutated amino acids. Reproduced from:
Individuals which were found to have a nonsense mutation (See Table 1)...