The Protein Text 



77 



Known replacements in homologous proteins are collected in Table II. 

 In the small sample we have (nineteen replacements), half recur twice or more, 

 suggesting strongly that the process, as observed, is not a random one. Unfor- 

 tunately, the sample is not unbiased. Certain replacements arc lethal or semi- 

 lethal (hemoglobin S, for example), and are, without doubt, selected against. 

 What we actually observe has therefore passed through the sieve of selection. 

 The direct genetic approach to this problem is tedious, because of the difficulty 

 of determining the phenotype (the amino acid sequence), and rapid progress 

 is scarcely to be expected. A much larger body of data on homologous proteins 

 may, however, enable us to reach a decision on whether the replacement 

 process is intrinsically restricted or not. 



An additional point emerges from a consideration of such protein mole- 

 cules as consist of more than one chain (Table III). It will be noted that there 



Table III. Terminal Residues of Proteins having more 



than one Peptide Chain 



(The exact number of chains is not indicated.) 



