Tissue kallikreins are a group of serine proteases that are found in many organs 
and biologic fluids. Tissue kallikrein genes (KLKs) are found on chromosome 
19q13.3-4 as a gene cluster encoding 15 different serine proteases. In skin, two 
tissue kallikrein proteins, hK5 and hK7, are expressed in the stratum corneum 
and are known to be involved in desquamation of corneocytes. The possible 
involvement of other kallikrein proteins has not been clarified, however, nor 
has the significance of each member in the serine protease activity of skin been 
delineated. In the study described here, we examined expression and localization 
of KLK mRNA in normal human skin by means of RT-PCR and in situ hybridization. 
Quantitative RT-PCR analysis showed abundant expression of KLK1 and KLK11 mRNA, 
moderate expression of KLK4, KLK5, KLK6, KLK7, and KLK13 mRNA, and low 
expression of KLK8 mRNA in normal human skin. For KLK4, KLK8, and KLK13 mRNA, 
splice variants were identified to be their major mRNA species. Two variants for 
KLK13 mRNA were novel. The amount of the serine protease inhibitor Kazal-type 5 
(SPINK5) mRNA was comparable to KLK1 and KLK11 mRNA. In situ hybridization 
revealed intense expression of all KLK mRNA studied except KLK12 mRNA in the 
stratum granulosum of normal epidermis, where SPINK5 mRNA coexisted. Excluding 
KLK13 mRNA, they are also expressed in hair sheath, eccrine sweat glands, and 
sebaceous glands. Coexpression of various KLK and SPINK5 mRNA suggests that 
their proteins are the candidates to balance and maintain serine protease 
activities in both the skin and appendages.