The construction of radiation hybrids always involves a selectable enzyme such as HPRT or TK. On the chromosome containing the selectable locus, at least one fragment containing the locus is necessary to form a viable clone. This requirement invalidates our model of fragment retention for the chromosome in question. The obvious amendment of the model is to condition on the event of retention of at least one fragment bearing the selectable locus. Discuss how this change of the model affects likelihood calculation via Baum’s forward and backward algorithms [16].