Specific expression of various RNA-binding protein (RNP) molecules is one of the critical factors that determine the development in Drosophila melanogaster and Caenorhabditis elegans. In the present study, we found a complementary DNA clone, LL55 encoding a putative RNP. Structural comparison of nucleotide and amino acid sequences demonstrate that LL55 encoding a nuclear protein with two RNA-binding consensus motifs and the common arginine-serine repeats, is similar to the sex-determining transformer-2 (tra-2) gene of Drosophila melanogaster. The expression of 2.3 kilobase (kb) and 1.5 kb LL55 messenger RNA (mRNA) is ubiquitous in murine organs and tissues, but it is selective in the early development of fetal life. It appears as early as day 10.5 and increases until day 15 of gestation in accordance with the expression of λ5 mRNA which is involved in murine pre-B cell generation. Taking account of the selective expression and the specific regulation of RNP molecules, the finding of LL55 RNP provides a new outlook for the generation and function of the immune system
