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This happens/happen when TE sequences are acquired as exon by host gene or mRNA, this process is called as exonization. On some occasions, their insertion occurs/occur in exon which is tolerated by natural selection without affecting the function of the host

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happens, occurs

This happens when TE sequences are acquired as an exon by a host gene or mRNA. This process is called as exonization. On some occasions, their insertion occurs in an exon which is tolerated by natural selection without affecting the function of the host.

I had to guess at some of that. I did not know whether more than one TE sequence can be acquired together as a single exon, which is what it says, or if you meant that one TE sequence becomes one distinct exon, in which case it should read "This happens when a TE sequence is acquired as an exon by a host gene or mRNA. This process is called as exonization. On some occasions, the insertion occurs …". Also, I did not know whether the word "host" was meant to apply to both "gene" and "mRNA". If so, it should read "a host gene or host mRNA …".