An md5sum (also known as md5 hash or checksum) is a digital "fingerprint"of a file.  It is a way to verify that the FCS file uploaded from your computer and the FCS file on Cytobank is the same, and it helps ensure data integrity. You'll see it on the successful upload page, one for each file uploaded.

To check md5sum for a file using a Mac:

1. In Finder, browse to /Applications/Utilities.
2. Double-click on the Terminal icon. A Terminal window will appear.
3. In the Terminal window, type: "openssl md5" (md5 followed by a space).
4. Drag the downloaded file from the Finder into the Terminal window.
5. Click in the Terminal window, press the Return key, and compare the checksum displayed to the screen to the one on Cytobank's upload page.

(Instructions modified from VMware's site)

To check md5sum for a file using Windows, you will have to download a program to do so. We do not have any particular program we recommend. This page gives some options, though we do not vouch for any recommended here: http://www.openoffice.org/dev_docs/using_md5sums.html#win Remember to check your Windows machine frequently for viruses with a good virus checker.