But the genetic factor supersedes the environmental factor in causing tooth agenesis, which is supported by molecular studies of familial autosomal dominant tooth agenesis associated with mutations in genes expressed in early tooth development such as paired box 9 (PAX9), muscle segment homeobox 1 (MSX1), axis inhibitor 2 (AXIN2), and ectodysplasin A (EDA). The gene discussed is AXIN2; the disease is tooth agenesis.