The occurrence of "off-target effects" is mainly due to the specificity of the DNA sequence to which the genome scissors are to "dock". If the DNA sequence is difficult for the genetic scissors to recognise and/or easily confused with another DNA sequence, off-target effects occur more frequently. Also, certain cells, for example, those from organs whose function is impaired due to disease, are sometimes more challenging to reach in genome editing procedures. This is because the editing tools must be transported to the cells to be modified, in particular with the aid of viral vectors. Still, this transport varies in efficiency depending on the type and location of the cells in the human body.