METHODS : Immunohistochemistry was utilized to detect the expression of differentiation products : basal cell cytokeratin ( CK 5 ) , luminal cell cytokeratin ( CK 18 ) , and prostatic steroid binding protein ( PBP ) , a ventral prostate specific marker indicative of secretory function in luminal cells . ^^^ Castration inhibited PBP protein expression , but did not affect the expression of luminal cell cytokeratin ( CK 18 ) protein . ^^^