Where on the chromosome?
The largest of your chromosomes, chromosome 1, is made up of about 300 million base pairs of DNA code. Even the smallest, chromosome 21, is about 50 million base pairs long. A single gene is usually only hundreds or thousands of base pairs long - just a fraction of the length of the whole chromosome. So gene hunters need to narrow down their search using a 'map' of the chromosome, with known 'landmarks' on it, like finding out which street in the area a building is in.