• Cryptic Splice Sites: Sequences within exons or introns that closely resemble the consensus sequence of authentic $5'$ donor (GU) or $3'$ acceptor (AG) splice sites, but are normally dormant and not used by the spliceosome under physiological conditions.
• Activation: If a normal authentic splice site is mutated or disrupted (e.g., in genetic diseases such as $\beta$-thalassemia), the spliceosome recognizes and cleaves at the cryptic splice site, leading to aberrant alternative splicing and defective mRNA products.

Correct option: (4) These are sequences within exons or introns that resemble consensus splicing signals but are not true splice sites.