Year ended December 31,
in USD ‘000
2021
2020
2019
External research and development costs
38,287
51,803
70,531
Staff costs (note 16)
18,788
19,643
24,556
Professional fees
12,175
3,994
7,072
Rents
60
22
21
Travel expenses
229
156
1,398
Patent registration costs
887
813
882
Depreciation
736
721
737
Other
3,466
2,566
1,914
Total operating expenses by nature
74,627
79,718
107,111
Depreciation expense has been allocated as follows:
Research and development expenses
441
447
429
General and administrative expenses
295
274
308
Total depreciation