Correct patterning of the antero-posterior axis of the embryonic trunk is dependent on spatiotemporally restricted Hox gene expression. In this study, we identified components of the Hoxd4 P1 promoter directing expression in neurally differentiating retinoic acid-treated P19 cells. We mapped three nucleosomes that are subsequently remodeled into an open chromatin state upon retinoic acid-induced Hoxd4 transcription. These nucleosomes spanned the Hoxd4 transcriptional start site in addition to a GC-rich positive regulatory element located 3 to the initiation site. We further identified two major cis-acting regulatory elements. An autoregulatory element was shown to recruit HOXD4 and its cofactor PBX1 and to positively regulate Hoxd4 expression in differentiating P19 cells. Conversely, the Polycomb group (PcG) protein Ying-Yang 1 (YY1) binds to an internucleosomal linker and represses Hoxd4 transcription before and during transcriptional activation. Sequential chromatin immunoprecipitation studies revealed that the PcG protein MEL18 was co-recruited with YY1 only in undifferentiated P19 cells, suggesting a role for MEL18 in silencing Hoxd4 transcription in undifferentiated P19 cells. This study links for the first time local chromatin remodeling events that take place during transcriptional activation with the dynamics of transcription factor association and DNA accessibility at a Hox regulatory region.Hox gene transcriptional activation marks the onset of an intricate series of events leading to proper embryonic patterning in all animals. The products of Hox genes, homeodomain-containing HOX transcription factors, are essential in specifying antero-posterior positional identity, hindbrain development, limb formation, and numerous additional morphogenetic and organogenetic events (1, 2). Given their crucial role in embryonic development, the genes encoding HOX proteins are highly conserved throughout the animal kingdom, and their expression is tightly regulated (3). In mammals, 39 Hox genes are organized into four clusters, each located on a different chromosome (1). Comparison of the clusters reveals 13 possible gene positions, although none of the clusters retains a full complement of 13 genes. Hox genes occupying the same positions are termed paralogs, sharing high sequence identity and functional redundancy. One can assign a 3Ј and a 5Ј end to a cluster since all genes are transcribed in the same direction. A unique feature of Hox gene clusters is a process termed "colinearity," correlating both the timing of transcriptional activation and the anterior expression borders with the position of a particular Hox gene along a cluster (4). Therefore, genes located more 3Ј are expressed earlier and have a more anterior expression border than genes located more 5Ј along the cluster. This observation and several other studies have led to the hypothesis that a sequential opening of chromatin, starting at the 3Ј end of a cluster and moving successively 5Ј, leads to the release of silencing, first at the 3Ј end, and seque...