 RNAi in vivo delivery to tissues still eludes science — especially to tissues deep within the body . RNAi delivery is only easily accessible to surface tissues such as the eye and respiratory tract . In these instances , siRNA has been used in direct contact with the tissue for transport . The resulting RNAi successfully focused on target genes . When delivering siRNA to deep tissues , the siRNA must be protected from nucleases , but targeting specific areas becomes the main difficulty . This difficulty has been combatted with high dosage levels of siRNA to ensure the tissues have been reached , however in these cases hepatotoxicity was reported . 
