The invention provides oligonucleotide probes that can be used to hybridize to a representation of nucleic acid sequences. Compositions containing the probes such as microarrays are also provided. The invention also provides methods of using these probes and compositions in therapeutic, diagnostic, and research applications. Systems and methods for using a word counting algorithm that can quickly and accurately count the number of times a particular string of characters (i.e., nucleotides) appears in a nucleotide sequence (e.g., a genome) are provided. This algorithm can be used to identify the oligonucleotide probes of the invention. The algorithm uses a transform of a genome and an auxiliary data structure to count the number of times a particular word occurs in the genome. (FIG.12)