Sequences are represented as strings containing A, T, G, C or A, U, G, C.
The positions returned by GenomeLookup["seq"] are given in the form {{"chr", strand}, {p1, p2}} where "chr" is a chromosome specification, strand is +1 or -1, and p1 and p2 are start and end positions.
Human chromosomes are specified as "Chromosome1", "ChromosomeX", "Mitochondrion", etc.
Bases are numbered in the 5' to 3' direction of the specified strand.
GenomeLookup[spec, "Count"] gives a count of the number of exact matches.