Targeted sodium-iodide symporter (NIS) gene transfer can be considered as a promising approach for diagnostics of specific types of cancer. For this purpose we used targeted polyplexes based on PEI-PEG-MC1SP block-copolymer containing MC1SP-peptide, a ligand specific for melanocortin receptor-1 (MC1R) overexpressed on melanoma cells. Targeted polyplexes demonstrated enhanced NIS gene transfer compared to non-targeted (lacking MC1SP) ones invitro. Using dorsal skinfold chamber and intravital microscopy we evaluated accumulation and microdistribution of quantum dot-labeled polyplexes in tumor and normal subcutaneous tissues up to 4h after intravenous injection. Polyplexes demonstrated significantly higher total accumulation in tumor tissue in comparison with subcutaneous ones (control). Targeted and non-targeted polyplexes extravasated and penetrated into the tumor tissue up to 20μm from the vessel walls. In contrast, in normal subcutaneous tissue polyplexes penetrated not more than 3μm from the vessel walls with the level of extravasated polyplexes 400-fold less than in tumor. Accumulated polyplexes in tumor tissue caused NIS gene expression. Subsequent123I-intravenous injection resulted in 6.8±1.1 and 4.5±0.8% ID/g (p