Ask an Embryologist: Genomic Mosaicism

A reader writes to ask:

In a recent ENV post, Casey Luskin observed that humans have a "few thousand" different cell types. Here is my simple question: Does the DNA sequence in one cell type differ from the sequence in another cell type in the same person? Do we really know that the DNA sequence in one person is the same all over his body? I am inclined to guess that the answer is: "No one is quite sure."

The simple answer is: We now know that there is considerable variation in DNA sequences...