p53
A gene that normally regulates the cell cycle and protects cells from damage to its genome. If a mutation occurs in this gene, cells can develop cancer. Individuals with Li-Fraumeni cancer syndrome have a defect in their p53 gene.

Pl-derived artificial chromosome (PAC)
One type of vector used to clone DNA fragments (100-to 300- kb insert size; average, 150 kb) in Escherichia coli cells. Based on bacteriophage (a virus) P1 genome.

Paleontology
The scientific study of fossils.

Palindrome
A sequence of DNA that reads the same in both directions from a central point of symmetry.

Paraphyletic group
A set of species containing an ancestral species together with some, but not all, of its descendants. The species included in the group are those that have continued to resemble the ancestor; the excluded species have evolved rapidly and no longer resemble their ancestor.

Pangaea
A supercontinent which began to break apart into the modern continents about 260 million years ago, causing the isolation (and separate evolution) of various groups of organisms from each other.