Intro
 Studying the human genome – the complete set of human genes – is a way of studying fundamental details about ourselves. A four-letter alphabet makes up the 3-billion-letter-long sequence of DNA that, divided into 23 pairs of chromosomes, inhabits the core of each cell in our bodies. Of course, people are not identical, and DNA sequences do differ subtly between individuals. The Human Genome Project is producing a representative sequence. Separate projects are charting variations in the sequence.
The representative sequence is a composite from several people who donated blood samples. Originally, close to 100 people volunteered to give blood and gave their informed consent, affirming that they agreed to the study of their DNA. No names were attached to the blood samples and ultimately scientists kept only a few. These measures ensured that the DNA sequence would remain anonymous; not even the donors knew whether their samples were actually used or not.
The Human Genome Project aims to read, letter by letter, the 3 billion units of human DNA. The Human Genome Project scientists began large-scale DNA sequencing in 1999. Before starting to sequence the human genome, they built maps of the human chromosomes and developed and refined techniques for analyzing DNA. With the tools in place, genome scientists took just a year to amass sequence covering more than 80% of the genome.
The human genome is a massive text. If the 3 billion letters, called "bases," that make up all the human DNA in a cell were printed in telephone books, the stack containing the whole genome would reach as high as the Washington monument.
To figure out the sequence of all the bases in the genome accurately, scientists need to read the 3 billion bases not just once, but at least 6 to 10 times. Sequencing reactions can only reveal the order of a few hundred letters of DNA at a time – amounting to a fraction of a page. Having many overlapping segments of sequence allows the genome to be puzzled back together into an intact whole.