The study investigates the genetic backgrounds of extreme childhood obesity (using saliva sample) and contains short online questionnaires on family history, phenotypical characteristics and hunger behavior. It is an observational study in a predefined cohort (subjects with data in CrescNet aged 0-18 with BMI measurement(s) above the 99.5th percentile). Recruitment will initially be from CrescNet data through the cooperating pediatric and adolescent medical institutions associated with these initiative of data collection in Germany.

In order to be able to investigate the genetic background of extreme obesity, current BMI developments above the 99.5 BMI percentile (extreme obesity) are selected in the existing CrescNet register. Pediatricians (submitting the data) will be informed about these cases. They will be ask to re-identify the patient using the pseudonym (CrescNet-ID) maintained in the registry and to contact the family with a request to participate in the study. The study physician of the study center at the Leipzig University Hospital will take over the study inclusion as well as all necessary clarifications and consents for the study and the genetic examination contained therein. Since the potential probands may live anywhere in Germany, telemedical contact between the study center and the family (interested in participation) will be used.

In addition to the above-mentioned study inclusion, the study physician is also responsible for instructing the family on the correct drawing, collection and sending of saliva samples to the study center, as well as instructing them on how to complete the hyperphagia questionnaire.

At this appointment, the proband is assigned an internal study identification number (INDICATOR-ID). Saliva sample and all questionnaires are assigned to this number.

Inclusion Criteria:

• Test subjects older 5 years:

BMI SDS between 0-5 years > 1.881 (= p97) BMI SDS between 5 and 18 >2.567 (=p99.5) Parents and subjects agree that the study information will be provided during a telemedical consultation by the study physician

Exclusion Criteria:

• Known cause of servere early obesity

Cohort from the registry CrescNet (NCT03072537) , data collection of body height and body weight in childhood by primary care pediatricians in Germany