Silencing and position-effect (PE) variegation (PEV), which is due to 
integration of viral vectors in heterochromatin regions, are considered 
significant obstacles to obtaining a consistent level of transgene expression in 
gene therapy. The inclusion of chromatin insulators into vectors has been 
proposed to counteract this position-dependent variegation of transgene 
expression. Here, we show that the sea urchin chromatin insulator, sns5, 
protects a recombinant gamma-retroviral vector from the negative influence of 
chromatin in erythroid milieu. This element increases the probability of vector 
expression at different chromosomal integration sites, which reduces both 
silencing and PEV. By chromatin immunoprecipitation (ChIP) analysis, we 
demonstrated the specific binding of GATA1 and OCT1 transcription factors and 
the enrichment of hyperacetylated nucleosomes to sns5 sequences. The results 
suggest that this new insulator is able to maintain a euchromatin state inside 
the provirus locus with mechanisms that are common to other characterized 
insulators. On the basis of its ability to function as barrier element in 
erythroid milieu and to bind the erythroid specific factor GATA1, the inclusion 
of sns5 insulator in viral vectors may be of practical benefit in gene transfer 
applications and, in particular, for gene therapy of erythroid disorders.