OXA are Oxacillinases which are a class of CHDL. Some OXA genes which are encoded in the plasmid of the bacteria(OXA-23) and other OXA genes like OXA 51 are chromosomally encoded genes. The numbers indicate the amino acid variants due to mutation or horizontal gene transfer between the different species of bacteria resulting in a new OXA gene. For example OXA-10 is a parental enzyme and mutation in these genes resulted in gene variants like OXA-13, OXA-14, OXA-16, OXA-17, OXA-19, OXA-28.