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Mixed Lineage Kinase 2 is a mammalian protein kinase that activates stress-activated protein kinases/c-jun N-terminal kinases (SAPK/JNKs) through direct phosphorylation of their upstream activator, SEK1/JNKK. We have examined expression of both MLK2 and SEK1/JNKK RNAs in the rat testis at various times during postnatal development and in isolated testicular cell populations. We also have used immunohistochemistry to examine MLK2 protein expression and localization in adult rat and mouse testis. In these analyses, we found rat MLK2 mRNA expression was first evident at a very low level on day 25 after birth and present from day 35 at much higher levels that continue into adulthood. In RNA from isolated cell types, a MLK2 transcript was detected in primary spermatocytes and round spermatids, but not in Leydig or Sertoli cells, MLK2 RNA was also absent from the testis of rats after induced cryptorchidism. SEK1/JNKK transcripts, on the other hand, were present at all stages of testicular development and in all cell types tested. In tissue sections from both adult rat and mouse testis, MLK2 immunoreactivity was present in the nucleus of primary and secondary spermatocytes and round spermatids within seminiferous tubules, but was absent from spermatogonia. These findings indicate the JNK pathway is most likely ubiquitous in rodent testicular cells, while the cell-specific pattern of MLK2 expression suggests that it may be involved in the regulation of processes specific to post-mitotic germ cells. Furthermore, the finding of MLK2 protein in the nucleus of spermatocytes and round spermatids indicates a role for MLK2 in regulation of nuclear events specific to germ cell development.