default 'legacy', it can be 'legacy' (for COSMIC v2 'SBS'),
'SBS', 'DBS', 'ID' and 'TSB' (for COSMIV v3.1 signatures).
For more specific details, it can also be 'SBS_hg19', 'SBS_hg38',
'SBS_mm9', 'SBS_mm10', 'DBS_hg19', 'DBS_hg38', 'DBS_mm9', 'DBS_mm10' to use
COSMIC v3 reference signatures from Alexandrov, Ludmil B., et al. (2020) (reference #1).
In addition, it can be one of "SBS_Nik_lab_Organ", "RS_Nik_lab_Organ",
"SBS_Nik_lab", "RS_Nik_lab" to refer reference signatures from
Degasperi, Andrea, et al. (2020) (reference #2).
Note: the signature profile for different genome builds are basically same.
And specific database (e.g. 'SBS_mm10') contains less signatures than all COSMIC
signatures (because some signatures are not detected from Alexandrov, Ludmil B., et al. (2020)).
For all available options, check the parameter setting.