Processed pseudogenes are formed by:
Processed or retrotransposed pseudogenes originate via retrotransposition of mature mRNA intermediate. Line-1 encoded endonuclease and reverse transcriptase recognize polyadenylated transcript, generating cDNA copy integrated into genome at new location. Hallmarks include absence of introns, presence of 3' polyA tract remnant, flanking direct repeats and lack of promoter, reflecting derivation from spliced message rather than DNA duplication. They are typically transcriptionally silent due to missing regulatory elements, accumulate mutations and serve as molecular fossils of ancestral gene expression patterns, abundant in mammalian genomes.
Ref: Vanin EF, Annu Rev Genet 1985: Processed Pseudogenes Formed by Reverse Transcription of mRNA