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Endogenous retroviruses (ERVs) are important retroelements that reside in host genomes. However, ERV expression patterns and regulatory mechanisms are poorly understood. In this study, chicken embryo fibroblasts (CEFs) and MSB1 cells infected with Marek’s disease virus (MDV) exhibited significantly increased expression of env from the endogenous retrovirus ALVE. In contrast, env expression was significantly lower in CEF and MSB1 cells infected with exogenous avian leukosis virus J (ALVJ) at the early infection stage. Furthermore, env was found to be ubiquitously expressed in various chicken tissues, with high expression in certain tissues at 2 days of age and low levels in most tissues, including immune organs (thymus, spleen and bursa) as well as the brain and heart, at 35 days of age. Sequence analysis revealed miR-155 target sites in env transcripts, which was verified using a firefly …