code: private boolean missingAllele(String alleleString) {
          return !((alleleString != null)
                  && !alleleString.replace("not specified", "")
                  .replace("NS", "")
                  .replace("NA", "")
                  .replace("na", "")
                  .replace("NULL", "")
                  .replace("null", "")
                  .replace(".", "").isEmpty());
      }
docs: Checks if a given allele is missing. An allele string can be empty (deletions, insertions), but cannot contain
  certain key words/values which would indicate that it's missing:
  {"not specified", "NS", "NA", "na", "NULL", "null", "."}
  @param alleleString
  @return true/false indicating whether the allele is missing or not.