Is HLA-B27 the ankylosing spondylitis susceptibility gene?

We studied a family in which the mother and her son had primary ankylosing spondylitis (AS). The mother carried the HLA-B27 allele, but did not pass this to her son. It is possible that the son is a coincidental, sporadic, non-B27 patient with spondylitis and the presence of B27 in the mother is relevant to the disease. Alternatively, it is possible that B27 in the mother is coincidental and the non-B27 haplotype, inherited by the son, carries the disease susceptibility gene. The results would then support the hypothesis that the B27 antigen may not be the AS gene, but rather a marker for another closely linked AS gene.