YY1 suppresses Igκ expression in HBL‐1 cells. (a, b) Compared with that in the control HBL‐1 cells (CON), the cells over‐expressing YY1 (YY1OE) expressed lower levels of Igκ, as indicated by real‐time PCR or Western blot. (c, d) Knocking down YY1 using small interfering RNA (siRNA) in HBL‐1 cells increased Igκ expression, as indicated by real‐time PCR and Western blot (*P < 0·05, **P < 0·01 compared with the control group).