By investigating rare recessive Mendelian or oligogenic inheritance patterns, we identify case-exclusive rare deleterious variation in known disease genes (<i>SI, LCT, ALDOB,</i> and <i>SLC5A1)</i> as well as candidate disease genes (<i>MGAM</i> and <i>SLC5A2),</i> providing potential evidence of monogenic or oligogenic inheritance in a subset of IBS cases. The gene discussed is MGAM; the disease is irritable bowel syndrome.