Translation is the process by which the information encoded within the mRNA is converted to protein. It represents a key step in the regulation of gene expression. Most of this regulation is exerted at the step of initiation (i.e. ribosome recruitment and start site selection). The 5’ Transcript Leader (5’ TL) of the mRNA contains multiple features that can regulate the translational readout, including RNA structure and upstream open reading frames (uORFs). These uORFs, via a process called “delayed reinitiation”, permit ribosomal access to multiple downstream initiation sites some of which have the potential to encode small bio-active proteins (<100 amino acids: referred to as SEPs). In my PhD thesis, we describe the identification and partial characterisation of a new 50 amino acid protein (SEP^53BP1) expressed from an internal ORF overlapping the CDS of 53BP1. Its expression is coupled to an alternative TP53BP1 gene promoter that expresses a V3 transcript variant with a short uORF within its 5’ TL. This uORF promotes initiation events at the internal SEP^53BP1 start codon. Our preliminary results suggest that one function of this protein is to interact with and regulate the activity of the proteasome.