Prostein, also known as P501S, is encoded by the Solute carrier family 45, member 3 (SLC45A3) gene, an androgen-regulated gene and a prostate specific marker expressed in prostatic glandular cells.  SLC45A3 is the second most common gene fusion partner in prostate cancers.  SLC45A3-ERG and SLC45A3-ELK4 fusion transcripts have been detected in prostate cancer.  Loss of prostein expression in SLC45A3-ERG tumors are significantly associated with high Gleason scores and poor clinical outcomes.   Prostein immunoreactivity is seen in the majority of primary and metastatic prostate carcinomas.  Staining for prostein in conjunction with PSA improves identification of prostatic origin in unknown primary lesions, when compared against prostein or PSA staining alone.