SAFB proteins regulate the splicing of transposable elements (TEs) in the human genome, preventing their exonization and maintaining genomic stability. These proteins bind to L1 elements and other TEs, particularly those with adenosine-rich sequences, and prevent their retrotransposition. SAFB proteins also suppress the exonization of nested genes, giant protein-coding cassette exons, and Tigger DNA transposons. Their activity is influenced by the genomic context, transcriptional direction, and RNA secondary structure. SAFB proteins are involved in the suppression of LTR/ERV elements in species where they are still active, such as mice and flies. In somatic cells, SAFB proteins are expressed at high levels, suppressing TE exonization, but their expression decreases in testes, where TE exonization is activated. This suggests that SAFB proteins function as a non-adaptive, RNA-based defense system against TEs in the soma, complementing the adaptive Piwi-interacting RNA pathway in the germline. SAFB proteins are also involved in the regulation of splicing of giant exons, which are more prone to exonization when SAFB is depleted. The adenosine bias in L1 ORFs is a key factor in SAFB binding and suppression. SAFB proteins compete with SR proteins for binding to GAA-rich sequences, preventing splicing interference. The function of SAFB proteins is conserved in mice and invertebrates, suggesting an ancestral role in regulating TE activity and nested gene expression. SAFB proteins are broadly expressed in the human body, but their expression drops in postmeiotic spermatids, allowing for TE exonization during spermatogenesis. This provides a controlled window for TE activity, enabling their continued contribution to genome evolution.SAFB proteins regulate the splicing of transposable elements (TEs) in the human genome, preventing their exonization and maintaining genomic stability. These proteins bind to L1 elements and other TEs, particularly those with adenosine-rich sequences, and prevent their retrotransposition. SAFB proteins also suppress the exonization of nested genes, giant protein-coding cassette exons, and Tigger DNA transposons. Their activity is influenced by the genomic context, transcriptional direction, and RNA secondary structure. SAFB proteins are involved in the suppression of LTR/ERV elements in species where they are still active, such as mice and flies. In somatic cells, SAFB proteins are expressed at high levels, suppressing TE exonization, but their expression decreases in testes, where TE exonization is activated. This suggests that SAFB proteins function as a non-adaptive, RNA-based defense system against TEs in the soma, complementing the adaptive Piwi-interacting RNA pathway in the germline. SAFB proteins are also involved in the regulation of splicing of giant exons, which are more prone to exonization when SAFB is depleted. The adenosine bias in L1 ORFs is a key factor in SAFB binding and suppression. SAFB proteins compete with SR proteins for binding to GAA-rich sequences, preventing splicing interference. The function of SAFB proteins is conserved in mice and invertebrates, suggesting an ancestral role in regulating TE activity and nested gene expression. SAFB proteins are broadly expressed in the human body, but their expression drops in postmeiotic spermatids, allowing for TE exonization during spermatogenesis. This provides a controlled window for TE activity, enabling their continued contribution to genome evolution.