Table 1. Standardised incidence ratios (SIRs) for breast cancer in 712 unaffected female blood relatives of 75 patients with ataxia telangiectasia (AT) from 66 Nordic families by familial relationship and probability of carrying an ATM mutation.

			Breast cancer
Female relative	No.	Person-years at risk	Obs	Exp	SIR	95% CI
All	712	22 892	34	19.51	1.7	1.2–2.4
Familial relationship
Mothera	64	1330	8	1.20	6.7	2.9–13
Other female relatives	648	21 562	26	18.31	1.4	0.9–2.1
Sister of proband	39	871	0	0.05	—	—
Grandmother	94	3502	8	4.87	1.6	0.7–3.2
Aunt	121	5488	2	3.24	0.6	0.1–2.2
Great-grandmother	89	2340	8	3.89	2.1	0.9–4.1
Grandmother's sister	118	4357	7	5.43	1.3	0.5–2.8
Female cousin	187	5004	1	0.83	1.2	0.0–6.7
Mutation carrier probabilityb
1c	72	1720	8	1.81	4.4	1.9–8.7
0.67; 0.50; 0.25	589	19 628	24	15.39	1.6	1.0–2.2
0.67; 0.50	252	9709	10	8.13	1.2	0.6–2.3
0.25	337	9919	14	7.33	1.9	1.0–3.2
Backgroundd	51	1545	2	2.31	0.9	0.1–3.2

Obs=observed cancers; Exp=expected cancers; CI=confidence interval.

^aThe mother was not known in each of two Swedish families.

^bThe estimated mutation carrier probability on an individual, according to the location in the pedigree, including information on consanguinity combined with the outcome of any relevant gene testing of relatives (see also text).

^cTwo mothers regarded as carriers of a wild-type allele were excluded from this group.

^dRelatives who married into consanguineous families, or relatives in branches of the family not involved in the gene transmission.