Sample size for logistic regression with multiple covariates

["Inman, Brant A. M.D." <Inman.Brant@mayo.edu>]

I am designing a mouse experiment for testing a new treatment for bladder cancer. The idea is to evaluate treatment effectiveness and treatment by gene interactions. The variables are:

Outcome (Y): binary (0/1); presence/absence of tumor
Covariate1 (X1):  categorical (0/1/2);  genotype of mouse (XX, X0, 00)
Covariate2 (X2):  binary (0/1); presence/absence of treatment

I am having trouble calculating the sample size for this experiment.  Does anyone know of an S-Plus or R function that has been written to calculate sample size for a multiple logistic regression situation?

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Dr. Brant A. Inman,  MD FRCSC
Urologic Oncology Fellow, Mayo Clinic College of Medicine
200 First Street SW
Rochester, MN   55905
Phone: (507) 284-1330
Fax:    (507) 284-4951
Email:  inman.brant@mayo.edu
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