The given statements are the steps in one type of gene therapy.
(i) Inject engineered cells into patients bone marrow.
(ii) Viral DNA carrying the normal allele inserts into chromosome.
(iii) Let retrovirus infect bone marrow cells that have been removed from patient and cultured.
(iv) Insert RNA version of normal allele into retrovirus.
The correct sequence is

Gene therapy is the introduction of normal genes into cells in place of missing or defective ones in order to correct genetic disorders. The correct sequence of the steps in gene therapy is

1. Insert RNA version of normal allele into retrovirus.
2. Let retrovirus infect bone marrow cells that have been removed from the patients and cultured.
3. Viral DNA carrying the normal gene insets into chromosome.
4. Inject engineered cells into patients bone marrow.