Final answer:
The most likely reading frames for finding significant BLASTX matches with a cDNA sequence are those that correspond with the known start codon 'ATG', starting the protein-coding region, as BLASTX does translate the sequence into all possible frames to find the best alignment.
Step-by-step explanation:
The most likely reading frames to find BLASTX matches with a cDNA sequence are those that align with the protein-coding sequence, which typically begins at the start codon (ATG). cDNA represents the coding strand; therefore, its sequence matches with the mRNA's orientation and codons. When using BLASTX to compare a cDNA sequence to protein databases, the reading frame that starts with an ATG codon is most likely to produce significant matches, representing a potential protein-coding region.
BLASTX translates the cDNA sequence into all possible reading frames, checking each frame for significant alignment with known protein sequences in the database. For a cDNA sequence, the reading frames where ATG corresponds to the commencement of coding for a Methionine in the protein are more probable to yield meaningful results, because these frames are likely to be in the correct orientation with the exon sequences in the genome.