Figure. Chromosomal organization of Hox genes and their expression in the lung.

Hox genes are organized in four clusters on different chromosomes, the Hox-a cluster resides on chromosome #6 (#7 in human), Hox-b on #11 (#17), Hox-c on #12 (#15), and Hox-d on #2 in human and mouse. The order of genes within a cluster is depicted in 5′ to 3′ orientation and paralogous genes are aligned vertically. Expression of Hox genes in the lung has been documented in human adult lung (capital letters), and rat and mouse lung at various embryonic stages (e), newborn (n) and adult (a) (small letters for rodents); data based on Northern or RT-PCR analysis only are represented by light shading, in situ hybridization or immunohistochemistry data are shaded darker. Open boxes represent genes that are not expressed in the developing lung in the mouse (such as Hoxb-1, b-7 and c-8) or have not been examined yet (for review, see 54, 55). The references are given in the text. During development, Hox genes are expressed sequentially in restricted domains along the anterior-posterior axis that reflect their respective position within a cluster (colinearity of position in cluster and expression domain): Hox genes located towards the 3′ end of the clusters are expressed more anteriorly while those located at the 5′ end are expressed in the posterior.