**2.6 Gene therapy**

Gene, the fundamental biological unit of heredity that constitutes an ordered sequence of nucleotides present in chromosomes. The functional aspect of a gene is to encode a protein or RNA molecule inherited from parents such as texture and color of the hair and eyes. Any kind of alterations/mutations in a gene sequence can lead to abnormal functionality of the genes. Gene therapy is a modern type of experimental technique in the medical field which involves rectifying the non-functional or malfunctioning of genes by replacing them with healthy and functional genes. Several approaches have been implemented by researchers in terms of correcting a mutated gene with a healthy copy of the gene or by inactivating the mutated gene causing disease. It has been widely studied for various diseases such as immune deficiency, blood disorders, eye problems, metabolic disorders, regeneration of nerve cells, and cancer [32]. The first case of gene therapy was discovered in the 1990s whereby a functional Adenosine Deaminase (ADA) gene was incorporated in the white blood cells of the patient, replacing the non-functional ADA [33]. This application led to interesting results with the immune systems and hence, was considered the most reliable technique.

There are two main methods for gene therapy such as- *Ex-vivo* gene therapy *and In-vivo* gene therapy. The former is the transfer of genes into patient cells outside the body and the latter one is the transfer of genes directly to cells inside the body. To carry this, several techniques are used like- direct or liposome-mediated injection of DNA, calcium phosphate transfection, electroporation, dendrimers, hybrid methods, retrovirus, and other viral vectors. Clinical conditions on which gene therapy has been applied are as follows:
