"Protective antigen" and "PA" refer to the protective antigen protein produced by the bacterium Bacillus anthracis (anthrax), and include any variants, isoforms and species homologs of anthrax protective antigen, which may be naturally expressed by the bacterium or recombinantly expressed. The terms "protective antigen" and "PA" refer to both the 83 kDa (PA83) and 63 kDa (PA63) forms of anthrax PA, unless a term is specifically limited to one form or the other.