We have isolated the gene for transition protein 2 (TNP2) from a human cosmid clone that contains the genes for protamines 1 and 2. A nucleotide sequence of 1776 bp that comprises 268 bp of the 5'-noncoding region, 400 bp of exon 1, 849 bp of an intron, 17 bp of exon 2, and 242 bp of the 3'-noncoding region was determined. A modified CAT box, a TATAA box, and two possible polyadenylation sites were identified. Transcripts in testicular RNA could be detected only by RT-PCR and RNase protection assays. By direct sequencing of the PCR products, a cDNA sequence was established. It can be deduced from these results that, in contrast to other mammalian genes, the human TNP2 gene is expressed at a very low level. The human gene differs from that of other mammalian species by the absence of a conserved GCCATCAC nucleotide sequence in the 3'-untranslated region. Since both protamine genes are known to be localized on chromosome 16p13.3, this chromosomal localization holds true for the human TNP2 gene as well. The genes for both protamines and TNP2 are arranged in a DNA stretch of 13 kb.