The androgen receptor gene (AR) contains a domain which includes a variable 
number of CAG sequences and alleles with low numbers of CAG repeats show high 
transactivation activity when complexed with testosterone. The ratio of 2nd and 
4th digit length (2D:4D) is negatively correlated with phenotypic effects of 
testosterone. Low numbers of CAG repeats and low 2D:4D are both associated with 
high sperm numbers and protection against breast cancer. This suggests that CAG 
number and 2D:4D are correlated i.e. low CAG number and low 2D:4D indicate high 
activation of androgen-responsive genes. Findings from AR studies predict that 
low 2D:4D will be associated with prostate and hepatocellular cancer, 
urolithiasis, ADHD, ankylosing spondylitis, spontaneous abortion, and polycystic 
ovaries, while high 2D:4D will be associated with motor neuron diseases and 
endometrial cancer. Findings from 2D:4D studies predict that short CAG length 
will be common in autism and Asperger's syndrome, while high numbers of CAG 
repeats will be found in men who are prone to early myocardial infarction.