Dental development begins between the sixth and eighth week of pregnancy. Teeth start to form inside the jaws long before moving to become visible inside the mouth in a process called eruption. The teeth of some children erupt earlier or later than others, and it appears that genetics play a part in regulating this process. The number of teeth in the mouth at age 15 months provides a simple measure of how far children have advanced in their dental development compared to other children the same age, and this data was used some years ago in ALSPAC to identify genetic variants which influence dental development. A collaborating research centre in the Netherlands has identified a genetic variant which they believe alters tooth eruption. Before publishing this finding they have asked us to check whether this variant has a similar effect on tooth eruption in ALSPAC, and cannot do this using the existing published results from ALSPAC as this particular genetic variant was not included in the previous study. We plan to test for association between this genetic variant and number of teeth at 15 months in ALSPAC.