show Abstracthide AbstractSex in birds is genetically determined, molecular mechanism of which is not well-understood. Their Z sex chromosome (chrZ) lacks whole chromosome inactivation as known for mammalian chrX. To investigate the extent of chrZ dosage compensation and its role in somatic cell's sex specification, we used a highly-quantitative method and analyzed transcriptional activities of male and female fibroblasts from seven birds. Our data indicate for the first time that ¾ of chrZ genes are strictly compensated, similar to that observed in chrX. We also describe non-compensated chrZ genes and identify Ribosomal Protein S6 (RPS6) as a candidate for universal, sex-dimorphic genes in birds. Overall design: Embryonic fibroblast cultures for seven bird species (chicken, quail, turkey, peacock, duck, zebra finch, and emu) obtained from males and females. At least two replicates.