Depends on what you are looking for. Often, there can be multiple entries of the same protein in PDB, each corresponding to a different organism. Besides, some structures (depending on how difficult it is to crystallize the protein) may have missing domains, poor resolution and even modified residues. You will have to be very specific in what structure you decide to select before performing docking. If you have the native/wildtype sequence for the BRCA1 protein, you can perform a BLAST against the PDB database and look at the IDs showing high sequence identity and coverage.

BRCA1 is a tricky one ( see https://www.uniprot.org/uniprotkb/P38398/entry#structure for a summary) - It is a very large protein (1863 AA for the canonical sequence P38398-1 of human BRCA1 ) with various isoforms. Available structures cover only a small fraction of the sequence (mainly residues 1646-1859, at the C-terminal end of the protein), and even alphafold did not give a structure prediction for most of the rest of the sequence. This indicates that much of the sequence may be natively unfolded (see "Structure" part of the UniProt record for a full list of structures mapping to this specific UniProt record). On the other hand, BRCA1 interacts with man other proteins, and parts of the interacting sequences may show induced structure in those complexes. If you simply do a text search for "BRCA1" in the pdb, you will retrieve many structures that do not contain any part of the actual BRCA1, but are known interaction partners of BRCA1. If you do a sequence search with the full length sequence of P38398, you are down to 91 structures, 65 of which are of a human protein, less than half of the 91 structures are actually BRCA1 fragments.. There are many structures containing mutations, because there are several known mutations of this gene that have been shown to significantly increase the risk of getting breast cancer https://www.uniprot.org/uniprotkb/P38398/entry#disease_variants , therefore it is of interest how these mutations affect the local structure and the interactions with other proteins. See the titles, abstracts and associated publications for the individual structures to see why these mutations were studied.

Choosing the appropriate protein structure for protein-ligand docking can be a challenging task, especially when dealing with cancer genes like BRCA1. Here are a few factors to consider when selecting a protein structure:

It's also worth mentioning that you can use multiple protein structures for the same gene, with different mutations or ligand association, to compare the results and to get a better understanding of the protein-ligand interactions.

In general, the best strategy is to use multiple protein structures from the PDB and to run a series of docking experiments to compare the results.

I hope this information helps you in your research. Good luck!