Embryonic stem (ES) cells are important developmental cells that appear very early during development and subsequently give rise to all the cell lineages of the future adult organism. In these cells a limited subset of transcription factors is expressed that are well conserved among species and essential for the fate of the stem cell. The transcriptome analysis of ES cells from chicken has revealed a gene family, cENS, that is specifically expressed in ES cells and in early embryos and is repressed during the differentiation process. This family is characterized by displaying retroviral structures and shares no homology with other species' genes. These characteristics are probably not restricted to the chicken genome and raise the question of whether similar genes are present and have been maintained in other species. We have examined the different copies of this gene in the sequenced chicken genome to investigate its dynamics and its evolution. We have distinguished two groups of cENS-related copies. The first group, resulting from recent transposition events, contains the transcribed ENS-1 and ENS-3 plus copies subjected to negative selection pressures. The second group contains degenerate copies that were integrated into the genome earlier. Comparison with copies previously isolated from three Galliformes showed that they are also subjected to selection pressures. We also detected numerous solo-LTRs containing the ENS-1 promoter that may control the expression of host genes. Taken together, these findings suggest a function sustained by a neogene of retroviral origin during the early stages of chicken development.