BMC Cancer 2008, 8:130 http://www.biomedcentral.com/1471-2407/8/130 tion with breast cancer status. For these analyses, PR was dichotomized (<5%, ≥ 5%) and the two uppermost cate- gories for ER were collapsed to ensure sufficient numbers in the cells for the analysis. All analyses were done using SAS (SAS Institute, Cary, NC, USA). A p < 0.05 was used to indicate statistical significance. Results When the 206 cases and 319 controls, the subset of sub- jects with measurements for ER and PR in nonneoplastic tissue, were compared with the total group of 267 cases Table 1: Characteristics of cases and controls, by study site and 400 controls who had completed a questionnaire, it was found that more subjects from Toronto (27.1%) had missing receptor results than from Kingston (11.8%); this was observed for both cases and controls. Characteristics of the cases and controls included in this analysis with respect to covariates are shown in Table 1. In both study sites, cases tended to be older than controls, have a slightly higher fat intake, and have smoked more ciga- rettes. A lower proportion of cases than controls was pre- menopausal, had never breast fed, reported current use of exogenous hormones, had a family history of breast can- Toronto Kingston Controls (n = 200) Cases (n = 101) Controls (n = 119) Cases (n = 105) ER, estrogen receptor; PR, progesterone receptor; SD, standard deviation.aInformation for BMI is missing for 3 subjects, energy, fat and alcohol consumption is missing for 4 subjects, cumulative smoking is missing for 4 subjects, and mammographic density is missing for 166 subjects. Characteristic Age, mean (SD), y Breast feeding, mean (SD), months Body mass indexa, mean (SD), kg/m2 Energy intakea, mean (SD), kcal/day Fat intakea, mean (SD), g/day Smokinga, mean (SD), pack-years Mammographic densitya, mean (SD), % Menopausal status, n (%) Gravid, n (%) Age last breast fed, n (%) never <30 ≥30 Oral contraceptives at questionnaire, n (%) Hormone therapy at questionnaire, n (%) Family history, n (%) Alcohol consumed >1 drink/weeka, n (%) Current smoking, n (%) Invasive cancer, n (%) Tumor steroid receptor status, n (%) ER+/PR+ ER+/PR- ER-/PR+ ER-/PR- Missing Atypical hyperplasia, n (%) Nonneoplastic cells staining for ER, n (%) <1% 1–5 6–10 11–33 34–66 >66 Nonneoplastic cells staining for PR, n (%) <1% 1–5 6–10 11–33 34–66 >66 44.9 5.7 23.3 957 33.3 7.8 41.0 54 149 105 42 53 12 26 83 103 49 - - 13 63 66 43 17 9 2 20 70 62 27 12 9 (9.6) (10.2) (3.9) (385) (14.8) (12.9) (18.7) (27.0) (74.5) (52.5) (21.0) (26.5) (6.0) (23.0) (41.5) (52.0) (24.5) (6.5) (31.5) (33.0) (21.5) (8.5) (4.5) (1.0) (10.0) (35.0) (31.0) (13.5) (6.0) (4.5) 54.3 5.3 24.3 991 35.1 9.0 35.8 59 78 45 24 32 8 8 35 44 18 88 55 10 7 24 5 - 21 23 27 17 9 4 22 27 31 12 9 0 (11.4) (9.1) (4.3) (307) (12.8) (13.2) (16.2) (58.4) (77.2) (44.6) (23.8) (31.7) (7.9) (7.9) (34.7) (44.0) (17.8) (87.1) (54.5) (9.9) (6.9) (23.8) (5.0) (20.8) (22.8) (26.7) (16.8) (8.9) (4.0) (21.8) (26.7) (30.7) (11.9) (8.9) (0.0) 56.1 6.5 25.7 981 37.4 10.9 26.5 79 106 65 32 22 1 27 41 52 25 - - 3 3 13 27 37 28 11 16 39 47 10 7 0 (11.8) (15.9) (4.5) (385) (17.9) (15.9) (17.6) (66.4) (89.1) (54. 6) (26.9) (18.5) (0.8) (22.7) (34.5) (43.7) (21.0) (2.5) (2.5) (12.4) (22.7) (31.1) (23.5) (9.2) (13.4) (32.8) (39.5) (8.4) (5.9) (0.0) 60.6 6.5 25.4 1121 43.3 13.4 26.0 81 93 46 26 33 1 10 42 37 22 94 59 14 4 20 8 - 2 4 24 23 25 27 16 36 31 12 8 2 (11.4) (9.5) (4.7) (374) (16.3) (17.1) (17.3) (77.1) (88.6) (43.8) (24.8) (31.4) (1.0) (9.5) (40.0) (35.6) (21.0) (89.5) (56.2) (13.3) (3.8) (19.0) (7.6) (1.9) (3.8) (22.9) (21.9) (23.8) (25.7) (15.2) (34.3) (29.5) (11.4) (7.6) (1.9) Page 4 of 9 (page number not for citation purposes)