A chromosome, and therefore the genes that are on the chromosome, is made of a chemical called deoxyribonucleic acid, or DNA. The DNA is composed of four chemical components called bases. The four bases are adenine, thymine, cytosine, and guanine. These components are symbolized by using one of 4 letters (A, T, C, or G).

DNA is made of two strands, each composed of a long string of the A’s, T’s, C’s, and G’s. The two strands run parallel to one another. In fact, the structure of DNA is often described as a twisted ladder. To create the ladder’s rungs, each of the bases on one strand is paired to a base on the other strand. Chromosome 1 is the largest human chromosome, spanning about 247 million base pairs and representing approximately 8 percent of the total DNA in cells.

So when we look at the question above, the 2.5Mb size tells us that the deletion is 2.5 million base pairs.  Knowing this information, we can examine the genes in the region that are deleted, but this gives us very little information for predicting clinical or medical problems.