Cell. karyotypes invariably comprise microchromosomes (MICs) and macrochromosomes (MACs) (Bloom et al. 1993). In the chicken, MICs 11C39 constitute approximately one-quarter of the genome and are cytologically indistinguishable from LH-RH, human each other because of their small size (Bloom et al. 1993). MICs consist of GC-rich DNA (Auer et al. 1987), are enriched for repeated sequences (Stefos and Arrighi 1974; Matzke et al. 1990), and contain heterochromatin from the criterion of C banding (Schmid et al. 1989). About two-thirds of the chicken genome (65%) is found on MACs 1C6 (Stubblefield and Orro 1982), which are the only poultry chromosomes larger than the smallest human being chromosomes. Chromosomes 7C10 are intermediate in size and have been arbitrarily designated as larger MICs (Bloom et al. 1993). In contrast to the MIC group, MACs display only faint C banding caused by HSNIK heterochromatin in centromeric and telomeric areas. However, R bands, which are associated with transcriptionally active DNA, will also be faint on MACs, having lower intensity and contrast than those of mammalian chromosomes (Schmid et al. 1989). MACs contain much AT-rich DNA (Auer et al. 1987) and, unlike MICs, are relatively rich in (CA)microsatellite repeats (Primmer et al. 1997). MICs were originally thought to be genetically inert (Newcomer 1955), but are now known to be bona fide chromosomes that are managed at a constant number and have conserved telomere sequences (Solovei et al. 1994). Many genes have recently been assigned microchromosomal locations (Bloom and Bacon 1985; Dominguez-Steglich et al. 1990, 1992a, 1992b, 1993; Jones et al. 1997). However, published gene-mapping data remains biased in favor of macrochromosomal projects (Burt et al. 1995). As with other vertebrates, the majority of genes in chicken possess CpG islands (CGIs) (Cooper et al. 1983; McQueen et al. 1996). CGIs are associated with the promoters of genes and may become differentiated from bulk DNA by their high GC content material, lack of depletion of the dinucleotide CpG, and lack of methylation (for review, observe Cross and Bird 1995). It is possible to exploit these characteristics for purification of CGIs from bulk DNA (Mix et al. 1994). In situ hybridization having a chicken CGI library constructed by this technique indicated that CGIs are concentrated on MICs. MACs by contrast were relatively CGI-poor by this criterion (McQueen et al. 1996). The implication that the majority of genes are localized on MICs was in contrast to mapping data and suggested that mapping data was biased towards MACs for technical reasons. The results led us to forecast that gene denseness on chicken MICs is similar to that of the pufferfish offers proven to be an excellent model for the study of vertebrate gene business because of its compact genome (Elgar 1996). Should chicken MICs reveal a similarly high concentration of LH-RH, human genes, then they may also serve as a useful model system for the study of larger vertebrate genomes. Reduced intron size of chicken genes relative to mammalian homologs, as seen for genes, is compatible with this idea (Hughes and Hughes 1995; Riegert et al. 1996). With this study we have analyzed the distribution of acetylated histone H4 in the chicken genome. Improved acetylation of the amino-terminus of histone H4 is definitely observed in transcriptionally active areas (for review, observe Turner 1993; Wade et al. 1997). For example, the distribution of acetylated H4 in human being and hamster chromosomes offers been shown to be nonrandom, with hyperacetylation of gene-rich R bands (Jeppesen et al. 1992), and hypoacetylation of heterochromatic domains (Jeppesen and Turner 1993). Histone H4 in CGI chromatin is known to be hyperacetylated compared with bulk chromatin (Tazi and Bird 1990). Acetylation studies therefore provide a method for visualizing regions of high gene content that is independent of sequence characteristics. Our study also includes a reinvestigation of the time of MIC replication during S phase. In mammals, transcriptionally active DNA is definitely early-replicating (Holmquist 1987) and CGI-rich areas have been shown to replicate early in human being chromosomes (Craig and Bickmore 1994). Assuming that transcription and early replication will also be correlated in the chicken, it is expected that microchromosomal DNA should be early-replicating. Earlier studies give an inconsistent picture with LH-RH, human respect to MICs. Schmid et al. (1989) concluded that MIC replication happens mainly in the.