Out of 851 soybean accessions from Vietnam, China and Japan analyzed for 7S b-subunit variants, a new b-reduced subunit line with normal growth was collected from the Mekong Delta, Vietnam. Protein of the 'b-reduced' line is composed of b-reduced and extremely low-b types. (a-null + b-reduced) type and b-null (or extremely low-b) type were screened in the progeny seeds of the Japanese mutant '(a + b) null' line. Therefore, recombination between a-and b-subunits was identified. By comparing the nucleotide sequence of the partial b-subunit gene of Enrei (standard), Mo-shi-dou Gong 503 (a + b low), b-5 33 seed (b-reduced), b-5 7 seed (extremely low-b), and (a + b)-null 16 ((a + b) null) seed, we found that the base 'T' at 166 bp, in Enrei changed to 'G' in Mo-shi-dou Gong 503, b-5 33 and b-5 7 . Using the (a + b)-null 16 individual as template, a distinct 305 bp b-subunit gene fragment was identified, instead of a 285 bp fragment.Abbreviations: bp -base pair; kDa -kilo dalton