SPHK1 and colitis: Another difficulty with the Alvarez interpretation of their data is that Sphk1-/- mice are viable and relatively normal (Allende et al., 2004), while Traf2 knock-outs on a C57BL/6 background die before or shortly after birth (Yeh et al., 1997) and on a BALB/c background develop severe colitis and die within three weeks after birth (Piao et al., 2011).