Leaky scanning provides a model for the observation that one mRNA is capable of producing two proteins, as seen in the HIV virus, which employs this mechanism to produce many required proteins. The first AUG is 'sometimes' bypassed by the 40S ribosomal subunit and initiation occurs at the second initiation codon. Therefore, two reading frames, distinguished by whether the initial segment, between the first and second AUG codons is read or not, depicts the resulting protein (Kozak 1999).
Possible explanations for this occurrence, is the absence of a strong promoting context around the initial AUG codon (Kozak 1991a). The observation of a train of elongating ribosomes progressing along an mRNA, in the 5' to 3' direction, would be able to physically bloc access to any downstream site, greatly reducing the validity of the leaky model. However, initiation at the second codon could be facilitated by the presence of the first AUG codon near the 5' end of the transcript, thereby being too close to become recognized efficiently (Kozak, 1995).
CUG, ACG or GUG, in a good surrounding context, are known to be capable of operating as initiation codons during leaky scanning. This observation can only occur when these codons are present upstream of the first AUG, as they are not capable of substituting as the sole start site (Kozak, 1991a). This results due to the presence of weak pairing between non-AUG codons in good context and Met-tRNA. Leaky scanning may be used to produce two proteins as mentioned above or may be a means of further regulating protein production (Kozak 1999).