The tissue kallikreins (KLKs) form a family of serine proteases that are involved in processing of polypeptide precursors and have important roles in a variety of physiologic and pathological processes. Common features of all tissue kallikrein genes identified to date in various species include a similar genomic organization of five exons, a conserved triad of amino acids for serine protease catalytic activity, and a signal peptide sequence encoded in the first exon. Here, we show that KLK4/KLK-L1/prostase/ARM1 (hereafter called KLK4) is the first significantly divergent member of the kallikrein family. The exon predicted to code for a signal peptide is absent in KLK4, which is likely to affect the function of the encoded protein. Green fluorescent protein (GFP)-tagged KLK4 has a distinct perinuclear localization, suggesting that its primary function is inside the cell, in contrast to the other tissue kallikreins characterized so far that have major extracellular functions. There are at least two differentially spliced, truncated variants of KLK4 that are either exclusively or predominantly localized to the nucleus when labeled with GFP. Furthermore, KLK4 expression is regulated by multiple hormones in prostate cancer cells and is deregulated in the androgen-independent phase of prostate cancer. These findings demonstrate that KLK4 is a unique member of the kallikrein family that may have a role in the progression of prostate cancer.