Recently, cloning of humans by nuclear transplantation has been proposed.  In this testimony I will focus on the scientific concerns about human cloning that have resulted from the experience with animal cloning.

1.      To date, five mammalian species (sheep, mice, goats, cows and pigs) have been cloned, however, survival of the nuclear clones has been uniformly low. The great majority of all clones (of all five species) die either at various stages of embryonic development, at birth, or soon after birth.  Most newborn clones are overweight and have an increased and dysfunctional placenta.  Those that survive the immediate perinatal period may die within days or weeks after birth with defects such as kidney or brain abnormalities, or with a defective immune system.  Even apparently healthy adult clones may have subtle defects that cannot be recognized in the animal.

2.      The most likely cause of abnormal clone development is faulty reprogramming of the genome.  This may lead to abnormal gene expression of any of the 30,000 genes residing in the animal.

3.      Faulty reprogramming does not lead to chromosomal or genetic alterations of the genome, so methods that are used in routine prenatal screening to detect chromosomal or genetic abnormalities in a fetus cannot detect these reprogramming errors.  There are no methods available now or in the foreseeable future to assess whether the genome of a cloned embryo has been correctly reprogrammed.

4.      The experience with animal cloning allows us to predict with a high degree of confidence that few cloned humans will survive to birth and of those, the majority will be abnormal.

The arguments given in this outline have been summarized in more detail in an article by Jaenisch and Wilmut that will be published in Science magazine on March 30, 2001.  A copy of the article will be available at the Committee meeting.