Immunoprecipitation and immunoblot recognition of PRB by antibodies to the unique N terminus of PRB. (A) hPRab2 was used to IP cell extract containing no PR, PRA, and PRB as indicated. (B) hPRab6 was used to IP cell extract containing no PR, PRA, and PRB. For A and B, the cell extract prior to IP was immunoblotted using AB52, which recognizes both PRA and PRB and is used as a control to demonstrate the presence of both isoforms of PR in the extract prior to IP.