Histone variants are nonallelic isoforms of canonical histones, and they are deposited, in contrast to canonical histones, in a replication-independent (RI) manner. RI deposition of H3.3, a histone variant from the H3.3 family, is mediated in mammals by distinct pathways involving either the histone regulator A (HIRA) complex or the death-associated protein (DAXX)/␣-thalassemia X-linked mental retardation protein (ATRX) complex. Here, we investigated the function of the Drosophila DAXX-like protein (DLP) by using both fly genetic approaches and protein biochemistry. DLP specifically interacts with H3.3 and shows a prominent localization on the base of the X chromosome, where it appears to act in concert with XNP, the Drosophila homolog of ATRX, in heterochromatin assembly and maintenance. The functional association between DLP and XNP is further supported by a series of experiments that illustrate genetic interactions and the DLP-XNP-dependent localization of specific chromosomal proteins. In addition, DLP both participates in the RI deposition of H3.3 and associates with anti-silencing factor 1 (ASF1). We suggest, in agreement with a recently proposed model, that DLP and ASF1 are part of a predeposition complex, which is recruited by XNP and is necessary to prevent DNA exposure in the nucleus.KEYWORDS DLP, heterochromatin, histone variant, H3.3, XNP, ASF1 I n the nucleus, DNA is packaged under the form of chromatin. The basic repeating unit of chromatin is the nucleosome, and the nucleosome core particle consists of approximately 146 bp of DNA wrapped around a histone octamer composed of two copies of each of the histones H2A, H2B, H3, and H4. (1, 2). Gene-rich domains are packaged into euchromatin, which is decondensed in interphase nuclei and is enriched for histone modifications characteristic of transcriptionally active regions. Moreover, euchromatin often exhibits high DNA accessibility. In contrast, gene-poor domains and repetitive sequences are packaged into condensed heterochromatin that carries histone modifications associated with transcriptional repression.In addition to the canonical core histones, cells express nonallelic-isoform histone variants (3). Histone variants have emerged as essential contributors to the regulation of chromatin structure, and they are involved in multiple processes, including chromatin stability, DNA repair, and transcriptional regulation. Canonical histones are almost exclusively expressed during the S phase of the cell cycle and are incorporated into chromatin in a DNA replication-dependent fashion, whereas replication-independent (RI) histone variants are expressed throughout the cell cycle.One of the best-studied histone variants is H3.3, which can replace the major species H3 (4). H3.3 is expressed and incorporated at all phases of the cell cycle (5). The available data suggest that H3.3 is a marker of active chromatin and associated with the