404. That’s an error.

The requested URL /bookmarks/mark?op=add&bkmk=https%3A//www.pnas.org/content/90/12/5796&title=A%20stationary-phase%20protein%20of%20Escherichia%20coli%20that%20affects%20the%20mode%20of%20association%20between%20the%20trp%20repressor%20protein%20and%20operator-bearing%20DNA was not found on this server. That’s all we know.