1. A plurality of nucleic acid molecules, wherein: (a) said plurality consists of N nucleic acid molecules; (b) each of said plurality of nucleic acid molecules has a nucleotide sequence that hybridizes specifically to a sequence in a genome of Z basepairs; and (c) at least P % of said plurality of nucleic acid molecules have (i) a length of K nucleotides; (ii) hybridizes specifically to at least one nucleic acid molecule present in or predicted to be present in a representation derived from said genome, said representation having no more than R % of the complexity of said genome; and (iii) no more than X exact matches of L 1 nucleotides to said genome and no fewer than Y exact matches of L 1 nucleotides to said genome; and wherein: (A N≧500; (B) Z≧1×10 8 ; (C) 300≧K≧30; (D) 70≧R≧0.001; (E) P=(N×R+(3×sigma))/N; (F) sigma is the squareroot of (N×R×(1-R)) (G) the integer closest to (log 4 (Z)+2)≧L 1 ≧the integer closest to log 4 (Z); (H) X is the integer closest to D 1 ×(K−L 1 +1); (I) Y is the integer closest to D 2 ×(K−L 1 +1); (J) 1.5≧D 1 ≧1; and (K) 1≧D 2 ≧0.5.