Finding one gene out of the possible tens of thousands of genes in the human—or any organism’s—genome is a difficult task, but the process is made easier if the protein product of the gene is known. For example, if a researcher is looking to find the gene for mouse hemoglobin, he or she would isolate the hemoglobin from mouse blood and determine the amino acid sequence. The amino acid sequence could then be used as a template to generate the nucleotide sequence. Working backward again, a complementary DNA probe to the sequence would be used to identify DNA molecules with the same sequence from the entire mouse genomic library. However, if the protein product is not known, the task is more difficult; for example, the difficulty of finding the susceptibility gene for late-onset Alzheimer’s disease in humans.