Each major type of virus can be further broken down into groups, which themselves can be subdivided into clades or subtypes. HIV-1 comprises groups M (main), O (outlier), and N (non-M or O). Screening tests in developed countries were originally developed to identify the most common HIV subtype in those regions – group M, clade B. In addition, third- and fourth-generation ELISA antibody tests are reliably able to detect group O virus, and the full range of group M subtypes