A large number of cDNA clones were isolated from an adult mouse testis cDNA library and partially sequenced. Sequence comparisons revealed that many of them displayed similarities to genes previously identified only in invertebrates and lower eukaryotes, including, e.g., one cDNA clone related to the Drosophila melanogaster suppressor of forked gene. Other cDNA clones were found to be related, but not identical, to rodent genes involved in a variety of cellular activities, e.g., signal transduction and tumor development. The RNA expression patterns of 258 cDNA clones were analyzed through use of Northern blot methodology. Forty-two novel cDNA clones were found to be expressed only in male germ cells, the majority of them during spermiogenesis. One male germ cell-specific mouse cDNA clone was found to be similar to tektin A1, a protein known to interact with the flagellar microtubules of sea urchin sperm.