Histone variants play important roles in the epigenetic regulation of genome function. The histone variant H2A.Z is evolutionarily conserved from yeast to vertebrates, and it has been reported to have multiple effects upon gene expression and insulation, and chromosome segregation. Recently two genes encoding H2A.Z were identified in the vertebrate genome. However, it is not yet clear whether the proteins transcribed from these genes are functionally distinct. To address this issue, we knocked out each gene individually in chicken DT40 cells. We found that two distinct proteins, H2A.Z-1 and H2A.Z-2, were produced from these genes, and that these proteins could be separated on a long SDS-PAGE gel. The two isoforms were deposited to a similar extent by the SRCAP chromatin-remodeling complex, suggesting redundancy to their function. However, cells lacking either one of the two isoforms exhibited distinct alterations in cell growth and gene expression, suggesting that the two isoforms have differential effects upon nucleo-some stability and chromatin structure. These findings provide insight into the molecular basis of the multiple functions of the H2A.Z gene products.