Substrate screening of the human protease HTRA1 in a placenta protein extract. The human serine protease high temperature requirement A1 (HTRA1) is highly expressed in the placental tissue, especially in the last trimester of gestation. This suggests that HTRA1 is involved in placental formation and function. With the aim of a better understanding of the role of HTRA1 in the placenta, candidate substrates were screened in a placenta protein extract using a gel-based mass spectrometric approach. Peptides were analysed using a nanoLC-ESI-IT-FTICR-MS instrument controlled by Xcalibur 2.07 software version (Thermo Scientific, Bremen). A by data-dependent acquisition method was used, where the five most intense precursor ions detected in the full MS scan (FTICR) were selected and fragmented in the IonTrap by collision induced dissociation (CID) (35% energy, 4 amu mass isolation width). Proteome Discoverer 1.3 and SEQUEST search engines (Thermo Scientific, Bremen) were used for data analysis. The MS/MS raw data were directly analysed with the Proteome Discoverer software using the following spectrum selector settings; minimum precursor mass 350 Da, maximum precursor mass 5000 Da, total intensity threshold 100, minimum peak count 10, signal-to-noise threshold 5 (FT-only). The identification searching parameters were: tryptic digestion, 2 missed cleavages, deamidated (N), oxidation (M) and propionamide (C) modifications, precursor mass tolerance 5 ppm and fragment mass tolerance 0.5 Da. The database UniProtKB/Swiss-Prot homo sapiens (September 2013, 20.267 Proteins) was chosen for protein identification. A list of contaminants was removed from this database, namely, Keratin, type I and type II cytoskeletal and trypsin. As discrimination criteria, protein identifications containing less than 2 peptides and SEQUEST scores lower than 40 were discarded.