Probability (y-axis) that a certain nucleotide (bottom x-axis) at a certain position in the promoter (top x-axis) occurs more than it actually occurs in the promoters classified as strong, assuming it follows a binomial distribution (the data for the weak class are similar). 16 promoters were classified as strong and 42 as weak. Only position/nucleotide combinations with more than 5 nucleotides and with a P-value less than 0.05 were retained. Thus a position/nucleotide combination with a high probability means that this nucleotide does not occur often in promoters classified as strong. The v nucleotide, v standing for void, at position 60 means no nucleotide present at that position.