The gene encoding the mouse cytotoxic T-lymphocyte-associated antigen 8 (CTLA8) has been cloned and its complete nucleotide (nt) sequence determined. Sequence and polymerase chain reaction (PCR) analysis indicated that the published CTLA8 sequence[Rouvier et al., J. Immunol. 150 (1993), 5445-5456] was of rat rather than mouse origin. The mouse CTLA8 gene contains two exons and one intron. The 5'flanking region contains several consensus motifs for binding to transcription factors and the 3' untranslated region (UTR) has AU-rich motifs associated with RNA instability. The putative exon sequences predict that the full-length mouse CTLA8 molecule contains 147 amino acids (aa) and shares 88% aa identity with rat CTLA8 and 57% aa identity to HSV13, an open reading frame (ORF) from herpesvirus saimiri (HVS).