Caudal regression syndrome, or sacral agenesis (or hypoplasia of the sacrum), is a rare birth defect.

In cases where more substantial areas of the spine are absent, there may be fused, webbed or smaller lower extremities and paralysis.

[2] The dominant inherited sacral agenesis (also referred to as Currarino syndrome) is very often correlated with a mutation in the Hb9 (also called HlxB9) gene (shown by Sally Ann Lynch, 1995, Nature Genetics).

[citation needed] Depending on the type of sacral agenesis, bowel or urinary bladder deficiencies may be present.

[citation needed] Before more comprehensive medical treatment was available, full amputation of the legs at the hip was often performed.

Prosthetics for children without substantial hip and trunk control is usually abandoned in favor of faster and easier wheelchair mobility as the child's weight and age increase.

In 2012, Spencer West, a man with sacral agenesis and both legs amputated, climbed Mount Kilimanjaro using only his hands.