Using mRNA differential display and cDNA library screening approaches we have identified differential gene expression of pecallex l - a mammalian homologue of pecanex gene frorn Drosophila - in the testes of the rat. Northern blot analyses showed that the transcript is only present in the germ line and not in the somatic cells of the testis, reaching its peak at the pachytene stage of the meiotic prophase. Moreover, nonradioactive in situ hybridization did not detect the expression of the gene in any cell type of the testis other than pachytene spermatocytes. Northern blot assays did not allow the detection of the transcript in nine other tissues. Remarkably, although pecanex exerts a neurogenic role in Drosophila, the transcript was not detectable by Northern blotting in the nervous tissue of adult rats, nor in the brain of neonate and embryonal stages. The protein product of the pecanex I gene was detected by immunoblotting in pachytene spermatocytes and round spermatids as well, but not in liver nor brain. From genomic analysis we conclude that, although only one pecanex gene exists in Drosophila, mammalian pecanex I belongs to a gene family with three related genes in different chromosomes. We speculate that pecallex I could play an important role in the testis, related to spermatogenesis. (c) 2005 Elsevier B.V All rights reserved.