The recently characterized human serine protease, Testisin, is expressed on 
premeiotic testicular germ cells and is a candidate type II tumor suppressor for 
testicular cancer. Here we report the cloning, characterization and expression 
of the gene encoding mouse Testisin, Prss21. The murine Testisin gene comprises 
six exons and five introns and spans approximately 5 kb of genomic DNA with an 
almost identical structure to the human Testisin gene, PRSS21. The gene was 
localized to murine chromosome 17 A3.3-B; a region syntenic with the location of 
PRSS21 on human chromosome 16p13.3. Northern blot analyses of RNA from a range 
of adult murine tissues demonstrated a 1.3 kb mRNA transcript present only in 
testis. The murine Testisin cDNA shares 65% identity with human Testisin cDNA 
and encodes a putative pre-pro-protein of 324 amino acids with 80% similarity to 
human Testisin. The predicted amino-acid sequence includes an N-terminal signal 
sequence of 27 amino acids, a 27 amino-acid pro-region, a 251 amino-acid 
catalytic domain typical of a serine protease with trypsin-like specificity, and 
a C-terminal hydrophobic extension which is predicted to function as a membrane 
anchor. Immunostaining for murine Testisin in mouse testis demonstrated specific 
staining in the cytoplasm and on the plasma membrane of round and elongating 
spermatids. Examination of murine Testisin mRNA expression in developing sperm 
confirmed that the onset of murine Testisin mRNA expression occurred at 
approximately day 18 after birth, corresponding to the appearance of spermatids 
in the testis, in contrast to the expression of human Testisin in spermatocytes. 
These data identify the murine ortholog to human Testisin and demonstrate that 
the murine Testisin gene is temporally regulated during murine spermatogenesis.