For use in IHC-Paraffin, the following protocol has been successfully employed: Placental tissues collected at 4-12 wk of gestation were fixed for 24 h at 4 C in 4% (vol/vol) paraformaldehyde, embedded in paraffin, and cut into 5-um sections. To control tissue integrity and select the most representative sections, every 10th section was stained with hematoxylin and eosin. Adjacent sections were stained using specific antibodies and the avidin-biotin immunoperoxidase detection method. Endogenous peroxidase activity was quenched by pretreatment with 3% (vol/vol) hydrogen peroxide in methanol for 30 min. Tissue sections were then washed in PBS and incubated with normal goat serum (10%) that served as a blocking agent for nonspecific binding.