Based on implementation of wiki page, so far there is no better way to constrain an integer into only two possible values. (Same as coordinatSystem) We propose add notes both in resource page and genomics implement page to help people understand choice of strand in different terminology systems: Proposed wording: There are lots of definition concerning with the Directionality of DNA or RNA. Here strand element in Sequence resource is defined to have constraints with value +1 and -1 , to indicate what is the direction for nucleotide series. In order to avoid confusion, we use +1 to express the "plus" strand (5' to 3') and -1 to express the "minus" strand (3' to 5'). Here is a very simple mapping that indicates which number the expression will be represented. +1 * -1* Watson Crick Sense Antisense positive negative Here is a small-scale example: if 5' GCGATATCGCAAA 3' is the data, then GC..AAA is plus strand while AAA..CG is the minus strand. Check preview at: https://bcl-lab.github.io/FHIR_CG_web/sequence.html#10.4.4.2