In gene therapy, how is the malfunctioning gene replaced? No plagarism! Solution Gene therapy is replacing defective genes with the genes having normal functionality. This treatment can be done at different stages.Therapy done at earliest stage of life (embyonic stage) is the most effective as it ensures gene replacement in whole lineage of cells having origin from therapeutic cell. Most common tool used for gene therapy are Viruses. These viruses are used as vector to carry desirable gene. Viruses are disarmed by removal of their harmful genes .Therapeutic gene is inserted in these viruses. While inserting therapeutic genes it is ensured that genetic material of viruses that has coding to control entry into host cells remains intact. These viruses are then exposed to target cells with malfunctioned genes.These viruses then transfer desired genes into the target cells.This step can be dne in situ or in vivo depending upon tissue being treated. Different types of viruses can be used for different mechanisms of DNA transfer. Challenges in gene therapy: 1. To ensure right positioning of desirable gene into host cell. 2. Sometimes gene may be over expressed. 3. Wrong position of gene in host cell may trigger changes in expression of DNA in vicinity..