Transcriptional repressor that represses lung-specific expression. Forms homodimers and heterodimers with FOXP1 and FOXP2. Dimerization is required for DNA-binding. Expressed in the adult heart, brain, spleen lung, liver, kidney and testes. Expressed predominantly in the lung and brain during embryogenesis. Expressed in the lung epithelium and the mesenchyme immediately adjacent to the epithelium from 10.5 dpc. At 9.5 dpc, expressed in the foregut endoderm but not in the heart. At 16.5 and 18.5 dpc, expressed in both the proximal and distal airway epithelium. Also expressed in the developing gut. In the hindgut, primarily expressed in epithelial cells of the intestine and stomach. Expressed in the brain in a dynamic pattern. At 14.5 dpc, expressed at high levels in the intermediate zone of the neopallial cortex with lower levels in the surrounding cells. By 16.5 dpc, no longer expressed in the intermediate zone, but still present in the surrounding cells of the neopallial cortex. The leucine-zipper is required for dimerization and transcriptional repression.