Recombinant DNA techniques appear to be a promising approach for identifying the genes responsible for influencing susceptibility to rheumatic disorders. The association of class I and class II RFLPs with AS and RA, respectively, provides further evidence that genes within the MHC play an important role in the etiology of these diseases. With regard to AS, the evidence gained by this approach suggests that a gene(s) in addition to B27 influences susceptibility to the disease. Furthermore, an RFLP has been identified that appears to distinguish clinical subtypes of the disease. Subsequently, one would now like to clone disease-associated RFLPs and determine their DNA sequence. Comparison of their sequence with the sequences of other MHC genes should reveal the identity of the gene contained within the fragment. Next, the clones can be restriction mapped and subclones obtained that could be used as probes in case-control studies to determine if they are more strongly associated with the respective diseases than the original probes. It can also be informative to construct genomic libraries of patients with AS and RA, respectively. These libraries can be screened with the original probe used to detect the RFLP as well as the subclones of the RFLP fragments to obtain additional clones that again could be used as probes in case-control studies to identify RFLPs more strongly associated with the respective disease. By obtaining a series of overlapping clones in this manner, one could 'walk' up and down the short arm of chromosome 6 searching for clones highly associated with the given disease. Those clones found to identify an RFLP highly associated with the disease could then be used as probes to determine whether the RFLP segregates with the disease in families with multiply-affected members. Identification of an RFLP that is strongly associated with the disease in case-control studies as well as concordant with the disease in family studies would provide presumptive evidence of a gene that plays a role in the etiology of the disease being studied. At this step, however, one could not be certain that the actual susceptibility gene had been identified as opposed to a tightly linked gene. Sequencing of clones containing the disease-specific RFLP would be needed to compare with known sequences of other genes. It should be noted that the chromosome 'walking' approach has proven effective in allowing investigators to clone the gene for retinoblastoma. Unfortunately, with multifactoral disorders such as the rheumatic diseases, it may be difficult to identify the actual gene(s). However, it is reasonable to speculate that at least one of the genes will be involved in some aspect of the immune response. This could be tested using transgenic mice or by investigating cells in culture transfected with the gene. The development of a probe that will identify an RFLP strongly associated with AS or RA is clearly a desirable goal. These genetic markers could more accurately predict risk for the occurrence of disease and, it is hoped, response to therapy and outcome.