What kind of mutation is present in the brca1 gene in the breast cancer families?

Chromosomal.: The brca-1 mutations account for around 5% of breast cancers. This maps to chromosome 17q21, and may be a transcription factor.

Loss of function: Brca 1 and 2 genes encode proteins than normally repair our dna. When the genes are mutated, the protein they normally make does not work correctly, leading to loss of the ability to fix other dna mutaions, and development of tumors.