The high growth mutation is a spontaneous deletion of 486,178 bp on Chr 10 with breakpoints in the second introns of suppressor of cytokine signaling 2 (Socs2) and plexin C1 (Plxnc1). It entirely eliminates Cradd (CASP2 and RIPK1 domain containing adaptor with death domain)/Raidd and results in generation of an 8.5 kb fusion transcript that joins the first two exons of Socs2 to the third exon of Plxnc1, which contains at least 20 exons. Of tissues analyzed, this fusion mRNA exhibits the highest expression in brain and much lower expression in liver and heart.