BY CHANCE ALONE-? 7-8% of the caucasian population is b27+. 2.5% of african-americans are b27+. The rarity of ALS is such that there has been no association defined. So 1/13 to 1/14 patients ( if caucasian) will be b27 positive. However, 1/3 b27+ patients have rheumatic issues, which might complicate the life-threatening ALS disease!