IntroductionAcute promyelocytic leukemia (APL) is a subgroup of acute myelogenous leukemia (AML) typified by the t(15;17) leading to fusion of the putative growth suppressor PML to the retinoic acid receptor ␣ (RAR␣), which has been implicated in normal myelopoiesis. 1 Recent studies suggest that PML-RAR␣, which retains the ligand (RA)-binding domain and functions as an aberrant retinoid receptor at physiological levels of RA, leads to repression of downstream target genes (reviewed by Lin et al 2 ). This is due to recruitment of nuclear corepressor/histone deacetylase (HDAC) complexes and DNA methyltransferases, 3 contributing to the differentiation block that characterizes the disease. However, the block can be overcome by pharmacologic levels of ligand in the form of all-trans retinoic acid (ATRA). The result is the induction of a conformational change in the fusion protein accompanied by binding of coactivators and release of corepressors, converting it from a repressor to a transcriptional activator. ATRA-induced degradation of PML-RAR␣ provides a further mechanism contributing to alleviation of the block in differentiation. 2 The precise molecular consequences of the presence of the PML-RAR␣ fusion and the changes that follow ATRA therapy are still far from clear. However, retinoids are potent morphogens and regulators in embryogenesis and organogenesis and have been shown to modulate HOX gene expression. Furthermore, the RAR␣ molecule has been hypothesized to act upstream of HOX genes. 4,5 HOX genes encode a subset of homeodomain-containing proteins, thought to bind DNA, which act as major regulators of embryogenesis and organogenesis. 6 HOX genes contain a highly conserved 183-base pair (bp) region termed the homeobox. [7][8][9] The genomes of all animals analyzed to date exhibit a distinct clustering of the homeobox genes, which are assumed to have arisen by duplication and divergence from a primordial HOX gene. In nonvertebrates one cluster exists-that is, HOM-C of insects and nematodes-whereas 4 clusters (HOX-A, B, C, and D) are present in mammals. The complex organization of the mammalian HOX gene cluster is based on 13 identifiable genes termed paralogs 1-13. Paralogs are distinguished by homeobox sequence similarity and position within the cluster. A total of 39 human HOX genes have been assigned to clusters A through D located at 4 different loci: 7p15, 17p21, 12q13, and 2q31, respectively. 10 During body patterning the expression of a particular HOM-C or HOX gene is related to its position within the chromosomethe colinearity rule. Recent work involving replacement transgenics suggests that the roles attributed to paralogous genes are the result of "quantitative modulations in gene expression" or gene dosage, 11 but the temporal order of expression of individual HOX genes throughout hematopoiesis remains to be elucidated.HOX genes play a significant role in both normal and dysregulated hematopoiesis. [12][13][14][15] In rare cases of AML involving a t(7; 11)(p15;p15) or t(2;11)(q31;p15) transloca...