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Table 2 Baseline distribution of study design and lifestyle risk factors by quartile of DNA methylationa

From: White blood cell DNA methylation and risk of breast cancer in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO)

  Q4 (high) Q3 Q2 Q1 (low)  
  N % N % N % N % p value
Age at entry (years)
 <59 85 39.5 67 31.5 76 37.3 73 34.0  
 60–64 66 30.7 79 37.1 60 29.4 76 35.4  
 65–69 46 21.4 48 22.5 52 25.5 49 22.9  
  > =70 18 8.4 19 8.9 16 7.8 17 7.9  
          0.75
Time at entry
  < =September 30,1997 134 62.3 125 58.7 114 55.9 132 61.4  
  > =October 1, 1997 81 37.7 88 41.3 90 44.1 83 38.6  
          0.53
Date of DNA extraction
 Pre-extracted DNA 69 32.1 69 32.4 70 34.3 90 41.9  
 Newly-extracted DNA 146 67.9 144 67.6 134 65.7 125 58.1  
          0.12
Race/ethnicity
 White, non-Hispanic 194 90.2 197 92.5 190 93.1 194 90.2  
 Other 21 9.8 16 7.5 14 6.9 21 9.8  
          0.60
Body mass index (kg/m2)
 Underweight-normal (15–24.9) 77 35.8 84 39.4 98 48.3 92 43.0  
 Overweight (25.0 − 29.9) 91 42.3 72 33.8 60 29.6 70 32.7  
 Obese (> = 30) 47 21.9 57 26.8 45 22.2 52 24.3  
          0.09
Age at menarche (years)
 <12 37 17.3 44 20.7 31 15.2 46 21.4  
 13 116 54.2 122 57.3 107 52.5 112 52.1  
  > =14 61 28.5 47 22.1 66 32.5 57 26.5  
          0.26
Number of live births
 None 18 8.4 20 9.4 16 7.8 25 11.6  
 1–2 66 30.7 65 30.5 66 32.4 73 34.0  
  > =3 131 60.9 128 60.1 122 59.8 117 54.4  
          0.75
Age at first birth (years)
 <20 36 16.8 32 15.0 24 11.8 35 16.3  
 20–24 101 47.2 94 44.1 100 49.3 95 44.2  
 25–29 47 22.0 54 25.4 48 23.7 39 18.1  
  > =30 12 5.6 13 6.1 15 7.4 21 9.8  
 Nulliparous 18 8.4 20 9.4 16 7.9 25 11.6  
          0.56
Type of menopause
 Natural 129 60.0 136 63.9 126 61.8 145 67.4  
 Bilateral oophorectomy 20 9.3 21 9.9 17 8.3 20 9.3  
 Other 66 30.7 56 26.3 61 29.9 50 23.3  
          0.67
Age at natural menopause (years)
 <50 53 41.1 46 33.8 41 32.8 46 31.7  
 50–54 56 43.4 69 50.7 66 52.8 72 49.7  
 55+ 20 15.5 21 15.4 18 14.4 27 18.6  
          0.65
History of benign breast disease
 No 133 62.7 143 69.4 130 64.4 140 66.7  
 Yes 79 37.3 63 30.6 72 35.6 70 33.3  
          0.51
Smoking status
 Never 115 53.4 118 55.4 115 56.4 138 64.2  
 Former 81 37.7 78 36.6 72 35.3 67 31.2  
 Current 19 8.8 17 8.0 17 8.3 10 4.7  
          0.31
Recent alcohol intake
 No 45 22.6 54 28.0 56 29.6 56 28.0  
 Yes 154 77.4 139 72.0 133 70.4 144 72.0  
Family history of breast cancer
 No 169 81.3 180 86.1 168 83.2 176 83.0  
 Yes 39 18.8 29 13.9 34 16.8 36 17.0  
          0.60
Hormone use
 Never 61 28.6 51 23.9 55 27.1 46 21.6  
 Former 32 15.0 33 15.5 24 11.8 36 16.9  
 Current 120 56.3 129 60.6 124 61.1 131 61.5  
          0.54
  1. aMissing values for cases and controls, respectively, were as follows: body mass index (1, 1), age at menarche (1, 0), number of livebirths (0, 2), age at first birth (0, 3), personal history of benign breast disease (11, 6), alcohol intake (35,31), family history of breast cancer (8, 8), and hormone use (2, 3). Q quartile