BTK and systemic mastocytosis: In order to test this hypothesis, we used flies that were mutant in two of the major immune defense pathways in flies, the Imd and the Toll pathways.40,41 As expected, all Imd mutant flies (PGRP-LCΔ5, imd1, and relishE20) succumb to any Sm infections (including ground control Sm) much more quickly than either of the wild-type fly lines (y1w* or w1118) as seen in Fig. 4a, since the Imd pathway is known to be critical for defense against infection with Gram-negative bacteria,42 such as Serratia marcescens.