Chromosome 4 of Drosophila melanogaster is an atypical autosome, exhibiting several features characteristic of the X chromosome. In the May 22 Proceedings of the National Academy of Sciences, Larsson et al. report the molecular cloning of Painting of fourth (Pof), which encodes a protein that specifically paints chromosome 4 (Proc Natl Acad Sci USA 2001, 98:6273-6278). The POF protein sequence contains a putative RNA-binding domain. POF protein decorates chromosome 4 by spreading out from an initiation site in the proximal region of the chromosome. In the related fruitfly species Drosophila busckii, POF paints the whole X chromosome, exclusively in males. This study supports the idea that there are similarities between the chromosome 4 autosome and the X chromosome. POF complexes may function like the Male Lethal Specific proteins, which regulate dosage-compensation of the X chromosome.

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