DirectoryEntry.Properties["memberOf"]

I'm curious about DirectoryEntry.Properties["memberOf"] and "Domain Users". Let's assume that I know a little about AD and that I can properly get the DirectoryEntry to any domain user. Now, every group that the user belongs to is in the memberOf property except for one exception. "Domain Users", is it a special case? Is it implied that if I get a valid DirectoryEntry that, of course, the user is a member of "Domain Users" and therefore "Domain Users" isn't in the memberOf property?