Drug hypersensitivity reactions and severe cutaneous adverse drug reactions, 
such as Stevens-Johnson syndrome and toxic epidermal necrolysis, are examples of 
serious adverse drug reactions mediated through a combination of metabolic and 
immunological mechanisms that could traditionally not have been predicted based 
on the pharmacological characteristics of the drug alone. The discovery of new 
associations between these syndromes and specific HLA has created the promise 
that risk for these reactions could be predicted through pharmacogenetic 
screening, thereby avoiding serious morbidity and mortality associated with 
these types of drug reactions. Despite this, several hurdles exist in the 
translation of these associations into pharmacogenetic tests that could be 
routinely used in the clinical setting. HLA-B*5701 screening to prevent abacavir 
hypersensitivity syndrome is an example of a test now in widespread routine 
clinical use in the developed world.