FZD4 and chronic obstructive pulmonary disease: In the present study, we report the downregulation of the Fzd4 gene, which codes for a WNT signaling receptor of the Frizzled (FZD) family; FZD4 was previously found downregulated in human and experimental mouse COPD tissue, and especially in primary human alveolar type II (ATII) cells from patients with COPD, together with reduced WNT/β-catenin signaling.