Final answer:
Intron sequences are spliced out of retroviral mRNA during the production of env products, leaving only the exons, which code for the necessary viral proteins.
Step-by-step explanation:
During the splicing event which allows env products to be made in retroviral mRNA, what is spliced out are the sequences known as introns. These introns are non-coding regions within pre-mRNA that do not contribute to the coding sequence for a protein. Splicing is a precise process carried out by the spliceosome that removes these introns, leaving only the coding regions, or exons, which are then rejoined to form the mature mRNA ready for translation into proteins, such as the ENV (envelope) protein crucial to the retrovirus's ability to infect a host cell.
Spliceosomes are complexes of proteins and RNA molecules that ensure the introns are removed and exons are rejoined with high accuracy. Specifically in the context of HIV and other retroviruses, the spliced mRNA then serves as a template for producing necessary viral proteins, including those that make up the viral envelope, which are essential for the virus to infect new host cells and spread the infection.