The HtrA2IFD allele (Zucker stock 3-5092) is an EMS-induced mutation that was identified by the TILLING method (Till BJ et al. (2003) Large-scale discovery of induced point mutations with high-throughput TILLING, Genome Research 13, 524-530).
These mutants contain a small in-frame deletion of 123 nt (from T231 to V271, 41 aa deleted), including a portion of the serine protease domain that covers the conserved catalytic S266 residue and another conserved S236 residue.