Reading frames:

Therefore the sequence of an mRNA molecule is read in sets of three nucleotides (codons)  from  the  5'  end,  it  can  be  read  in  three  possible  reading  frames it depending on that nucleotide is used as the first base of the first codon shown in the figure.

Figure:  Three potential reading frames for any given mRNA sequence depending on which nucleotide is 'read' first.

Generally, only one reading frame will produce a functional protein since the other two reading frames will involve various termination (Stop) codons. The correct reading frame is group in vivo through recognition by the ribosome  of  the  initiation  codon,  AUG,  at  the  begin  of  the  coding  sequence. Commonly one sequence of bases encodes only a single protein. Moreover, in some bacteriophage DNAs, various genes overlap, with every gene being in a different reading frame.  This industry of overlapping genes usually occurs when the genome size is smaller than can accommodate the genes necessary for phage structure and assembly using only one reading frame.