Display guidelines for Crossref DOIs - effective March 2017 use “https://doi.org/”.
However you often see DOI's listed like this: DOI: 10.1006/jmbi.1995.0238
Add http://dx.doi.org/ to the front to create a link that will resolve to the online document, database, image, etc.
URLs without the dx. also work: http://doi.org/
Note that DOIs are permanent and cannot be deleted. Another identifier schema, ARKs or Archival Resource Keys, can be used to track a resource in development before a decision is made as to whether or not the resource will be made permanent, at which point a DOI can be assigned. ARKs are also issued through the EZID service, to which WU subscribes fro DOI assignment. The local interface to the service does not yet include creation of ARKs.