Splicing: In genetic engineering, a set of foreign genes is spliced - inserted - into the middle of the DNA 'code words' (see DNA to find DNA moleculeout about the 'instructions': the genetic code). This splicing can mess up the normal coded instructions in the DNA. And that can go on to mess up how the cell works. No-one can know in advance what might happen and whether it might be hazardous. It is unpredictable. The insertion or splice could make the chromosome behave in a quite unexpected way. This does not happen in normal mating because the arrangement of the coded instructions does not change when the chromosomes of the father and mother combine. So when people claim that GE is more or less the same as natural mating (sexual reproduction), they are wrong. For more on this, see Physicians and Scientists for Responsible Application of Science and Technology