Alveoli form, in part, by subdivision (septation) of saccules constituting the gas exchange region of the immature lung. Rat lungs septate from postnatal day 4 through 14 but alveoli in male rats continue to form, by other means, until about age 44 days. In rats, we sought to identify genes involved in septation. Using differential display reverse transcription polymerase chain reaction, we cloned a cDNA, rA5D3, that had 1270 bp with an open reading frame encoding a putative polypeptide of 165 amino acids containing potential leucine zipper motifs and phosphorylation sites. Database searches indicated rA5D3 was a novel gene. rA5D3 RNA relative concentration increased 1.7-fold between postnatal days 4 and 8, decreased 4-fold between days 8 and 14, declined significantly thereafter but was still detected post alveolus formation. If the expression of rA5D3 protein resembles its RNA, its peak (postnatal day 8) is well after the onset of septation, suggesting a role other than the initiation of septation.