Table 6.

Probabilities That an Unaffected Child Who Has Been Tested after Birth and Has Been Found to Carry a Homozygous Absence of SMN1 Will Develop Type I, II, or III SMA, on the Basis of Number of SMN2 Copies

	Probability ofDeveloping SMA
Probability Type andNo. of SMN2 Copies	Type I	Type II	Type III
Prior probabilities	.51	.32	.17
Conditional probabilities (OR):a
One copy	1.00	<.001	<.001
Two copies	1.00	.044	.0145
Three copies	1.00	18.4	4.19
Four copies	1.00	14.7	156.7
Joint probabilities:
One copy	.51	<.00032	<.00017
Two copies	.51	.0141	.000255
Three copies	.51	5.89	.712
Four copies	.51	4.70	26.6
Posterior probabilities (%):
One copy	>99.9	<.1	<.1
Two copies	97.26	2.7	.04
Three copies	7.2	82.8	10.0
Four copies	1.6	14.8	83.6

^aThe OR for one copy was used as a reference and was fixed to unity.